A flash hider would be fine. although if you alter the SKS in any way, including adding/removing a muzzle device, it is no longer a C&R firearm. -not a big deal for most.
if you add any features that would make it illegal for import, ie. detachable mag, folding stock, pistol grip, it must meet 922r status and US parts would have to be added.
that is my understanding anyway.