AK-97 or Colt M23. I agree that it should have an external smartgun system.
Other ideas:
-- Folding stock, possibly cannibalized from a different weapon model altogether, depending on the armorer skill of the original owner
-- Sling, another avenue to add personality to the weapon and a must for any assault rifle as far as I'm concerned
-- Foregrip, fluff-wise very useful for CQC, and any extra RC is useful. Of course you could also go the grenade launcher route
-- Slide Mount, if the weapon's been tinkered with, it probably has one of these for ease of tinkering.
-- Gas Vent, as has been touched on, I think this is a solid choice
-- Bayonet, because you can fucking stab somebody with your rifle.