Sgt Reeves and the boys back from retirement
They can chose the orders and equipment cards of any Space Marine chapter. Campaign advancement is the same as Marines.
The scouts are fast but vulnerable to gunfire, so they get extra actions, cover and going prone, loosely based on Advanced Space Crusade.
Rules: A roll of 1 cannot kill a scout in H2H. A commander is a scout for rule purposes.
At the start of each turn, Roll 2 red dice to get the Squad's Extra Action points, each point may be used for an "Extra Action"
1) One reroll on one shooting attack (aimed fire attack), not overwatch.
2) Enter overwatch, may fire once during enemy turn when a moving enemy halts, before it attacks.
3) Throw a grenade (3x3 square, 2w) in addition to firing, not overwatch.
(You can make up more actions, such as first aid with a 1 in 6 chance of saving a marine killed on the previous turn)
You show a scout is in cover by placing it on the line between 2 squares. Unless an attacker can shoot at both squares, the scout is under cover from that attacker, and gets +1 to armor. This action costs 1 movement point. Next turn, the scout can start to move from either square.
Prone: Move 1 square, and turn mini over. Defender gets a +1 armor if shot at, may only roll 1 die in H2H.
Comment: The scouts are very different from Marines, and they borrow from Advanced Space Crusade, so that adds interest. They are worth shooting at by Orcs and Chaos Marines, since they are good at H2H. They can shoot more than Marines, although they have only 1 heavy weapon. The sniper can reveal a suspected dreadnought, and stay well away from the enemy.
You could use Eldar as scouts, the extra speed and skills make sense.
The scouts are easily available on EBay, and you can google the 3D printer files. "Scout Sniper" and "SM Scout" on TinkerCad. Also "Space Orc".