Check
HERE for some very good info on the legal issues with modifying a Yugo 59/66 SKS.
There is something called the "10 or less" game. If you want to add some features like a folding stock or detachable magazine you must replace enough parts to have 10 or less imported parts.
Just removing the bayonet and grenade launcher and gl sights doesn't make it illegal. It does remove it from C&R status and makes it necessary to comply with U.S. 922(r).
The basic bad mods are a detachable mag and pistol grip or folding stock. Since the carbine is imported these mods would require you to play the 10 or less game.
If you want to leave the fixed mag and issue stock then removing the gl, gl sights, night sights and bayonet won't make it illegal. It will just remove it from C&R status.
This weapon has caused a lot of confusion. Even the BATFE gives different answers to some questions, but these mods are the basic ones they have said is "OK".
The SKS board linked to above has thoroughly thrashed this subject and is very good on these questions. One guy there, Schwartz, is really on the ball as far as the legal issues are concerned. Post a question and he can confirm this info.
It seems they lost a lot of back posts during a board or server crash or something, so posting a new question may help more than a search of the site.
HTH,
Jim