To futher clarify Circuit's (correct) statements, an imported semi-auto is only truly "pre-ban" if you can prove it was imported before 1990, because in 1989, Bush Sr. signed a bill that modified the import restrictions of the GCA68, essentially banning the import (or later modification) of guns with "military features."

Since a post-90 SKS couldn't be imported with a flash suppressor, you can't install one unless you can PROVE that the gun was imported before the ban.  This is because the SKS is an imported rifle, and must meet the '89 laws as well as the '94 Krime Bill laws.

However, as Circuits points out, if you replace enough imported parts with domestic-made parts, the gun will cease to be an imported gun under the law, and will no longer have to comply with the '89 laws.  That's how all of those kit guns (AKs, FALs, G3s, CEMTEs, etc.) can have pistol grips; they use enough domestic parts that they aren't considered imports.

You'd still need to comply with the '94 rules, but with a fixed 10-round mag, you can add just about anything.

-Troy