Replacing Space Crusade Marines with something else.

In which simple cards modify the abilities of a Space Crusade Marine Squad, giving it the abilities of a different race or army. There are no new rules, except for the cards, so nothing to remember.

You may have some other Scfi Miniatures; you could take 5 of them, and replace 5 Space Marines with your Necrons, Daleks, Steampunk Aquanauts, or whatever. But you would need rules and cards for them, like Eldar Attack, so it would be a lot of work.
One simple approach is to say they are basically Blood Angels, but substitute new Order and Equipment cards for some of the Blood Angels cards. For example, you would have some kind of Berserker Order card and Equipment card, to allow your guys to make at least one rush into frenzied hand-to-hand combat.
The cards can be simple, even handwritten:

ORDER CARD Berserkers!
Entering an uncontrollable frenzy, Berserkers throw caution to the wind in close combat.
Each Squad member rolls exactly 2 red die for every hand-to-hand attack roll for this turn. Add 3 to movement allowance for the turn. During the enemy turn, the enemy may re-roll one die of every attack roll it makes. Discard this card after use.

This would be good for Khorne Berserkers, and also Orcs.

For Dark Eldar, or generic "Evil Dudes":

The Squad is compelled to feed their unholy appetites for other sentient beings' souls, a terrible desire called the Thirst.
If your Squad kills 2 living enemies on one turn (Not Android), each squad member gets to roll an extra red die on every attack roll in the next turn.

ORDER CARD Combat Drugs!
Combat Drugs turn your squad into frenzied killers.
For 1 turn, every squad member’s ranged attack gets 1 extra white die, every hand-to-hand attack gets an extra red die. Movement allowance is increased by 3 for the turn.
Discard this card after use.

Orks could use the Combat Drugs card too, except that you would call it WAAAGH!.

Special weapons can be abstracted out:

ORDER CARD Combi Weapons!
Combi-weapons are derived from the integration of multiple different weapon types into a single weapon.
Each non-heavy weapon squad member may fire a Plasma Rifle or a Flamer (if available) instead of their normal weapon this turn.
 Discard this card after use.

EQUIPMENT CARD Executioner Axe!
This is a rare master-crafted heavy bladed Power Axe.
If the Commander has an axe, he may roll 1 extra red die in hand-to-hand.

This is a rare master-crafted heavy bladed Power Axe.
The rocket launcher only affects 1 square, but it rolls 2 red and 1 white dice. This is a "free" card in addition to the existing Equipment Cards.

This is an advanced form of powered armour developed for close-quarters melee fighting.
All Squad Members have Armor of 3, and roll one red and one white die in Hand-to-hand.
All Squad members have a move of 5, even with heavy weapons.
This card replaces all other equipment cards.

This Chaos Marine of Tzeentch just has a Bolter, so we will give him a free Mark of Fear, because he has a funny thing on a stick:

The Commander holds a Battle Standard which terrifies the enemy.

Any enemy about to move into line of sight of the Commander must roll a white die. If the result is 0, the enemy unit may continue. On a 1 or 2 its turn ends, not entering line of sight of the Commander. Androids are immune to this card.

You can wallpaper over rules you don't like:

An electronic scanner gives a one second of warning of nearby non-human intelligence.
When a Genestealer first attacks, it does so with only 1 red die.

Different Species:

EQUIPMENT CARD Horde Mentality!
Your squad gangs up on enemy individuals, tearing them asunder.
If one of your squad loses or draws in hand-to-hand, another of your squad within 4 squares may take a free move and a free
 hand-to-hand attack on the enemy concerned.

You swap the extra cards with the existing Space Marine chapter cards. You may give a squad a free card as you see fit. You can buy an extra card if you reduce the Commander's health to 3. It is probably better to give your squad abilities that reflect their background than to be perfectly balanced.
The basic weapon will be a bolter, or a red and white dice for hand-to-hand only.
Heavy weapons will be the same as Space Crusade, but you can modify one of the weapons with a "free" card like the Laser Lance above.
The Commander will make a weapon choice that is the closest to his actual abilities. The Power Sword/Claw can be a tentacle or whatever.

You make up your own cards to suit your team.

ORDER CARD Overwatch!
Your squad stands ready to ambush.
Each Squad member may fire during this enemy turn, once every time an enemy becomes newly visible. 
Discard this card after use.

ORDER CARD Combined Fire!
Concentrate small arms fire on a particularly strong target.
All weapons may combine their dice rolls if they can all see the target. Not Heavy or Special Weapons. Discard this card after use.

Your squad takes cover from heavy fire.
Each Squad member may make an extra move at any time during the Enemy turn.
Discard this card after use.

EQUIPMENT CARD Mark of Courage!
The Leader holds a Battle Standard which inspires his Squad to greater things.
Any Squad member within 1 square of the Commander may re-roll 1 die on any dice roll they make.

ORDER CARD Outflank!
The Squad executes a well-rehearsed attack drill.
Each Squad member may move the full amount, fire and move the full amount again.
Discard this card after use.

EQUIPMENT CARD Artificer Armor!
Ancient armor technology gives increased freedom of movement in combat.
All weapons except heavy weapons and special weapons roll an extra white dice in hand-to-hand combat.
Each Squad member may move the full amount, fire and move the full amount again.

A light squad may go prone to make itself harder to hit.
If no one in the Squad moves during this turn, the Squad may declare "Prone", All Enemy may be force to re-roll 1 die on any ranged attack dice roll they make on a "Prone" squad. The Squad may be forced to re-roll 1 die on any dice roll they make in Hand-to-Hand combat if they have declared "Prone".

Flamethrowers burn an area for the rest of the turn.
The Plasma rifle is replaced by a Flamer. The Flamer is a Missile launcher, but you place a marker in all 9 squares affected. R
e-roll the dice again to attack any friend or enemy who moves into any marked square. Flame markers block LOS. Remove the markers at the end of the turn. If the Flamer is fired in one turn, it cannot be fired on the next turn.
Comment: If you can use the flamer every turn, you can just burn your way down the corridor until the Genestealer gets the flamer guy. Somewhat overpowered.

Senior Officers:

EQUIPMENT CARD Apothecary Commander!
The Apothecary retrieves gene-seed from slain Marines and tends to the wounded and maimed.
If a Marine is killed, and the Commander ends his turn adjacent to that marine, roll a white die. If the result is "2", the Marine is brought back to life at the end of the Marine turn. If the result is a "1", the gene-seed is recovered, so the alien player does not get the miniature, and gets no points from the kill, even though the miniature is removed from play. A "0" means the Marine dies normally.

EQUIPMENT CARD Chaplain Commander!
He leads from the fore, rejoicing in the righteous slaughter of his enemies.
Any 2 Marines Adjacent to the Commander may re-roll one die of their choice when attacking.

Dark Eldar and World Eaters example, plus blank cards:

Pistols: The Commander may have a cutting weapon and one hand and a pistol in the other. A simple pistol rule is that a pistol is the same as a full-size Bolter/Plasma Rifle/Melta, but the target gets a saving roll. Each target that is killed or reduced in life points by a pistol gets to roll a white die. If they get a 1 or a 2, the shot actually missed, 0 is a normal hit. A pistol is never a Heavy Weapon. 
Optionally, no saving roll, Commanders are just very good shots.
A flame pistol might have to be a 1-shot Heavy Flamer, since it holds a small amount of Promethium.