I'm a huge SKS fan and have done more reading that I care to mention about the laws. Based on what I've read, if you make ANY modification to a C&R gun (adding or removing parts) it automatically voids the C&R status, which is totally legal to do. However, because it's no longer a C&R gun it falls under 922r and must have no more than 10 imported parts. Basically you have to make it a non-imported gun by swapping in American made parts so as to get the total count of imported parts to 10 or less. As per the ATF, here is a a complete list of parts they recognize
(1) Frames, receivers, receiver castings, forgings, or castings.
(2) Barrels.
(3) Barrel extensions.
(4) Mounting blocks (trunnions).
(5) Muzzle attachments.
(6) Bolts.
(7) Bolt carriers.
(8) Operating rods.
(9) Gas pistons.
(10) Trigger housings.
(11) Triggers.
(12) Hammers.
(13) Sears.
(14) Disconnectors.
(15) Buttstocks.
(16) Pistol grips.
(17) Forearms, handguards.
(18) Magazine bodies.
(19) Followers.
(20) Floor plates.
There are some parts on that list that are not applicable to the SKS so you don't need to worry about them:
Barrel Extensions
Mounting Blocks
Operating Rod
Pistol Grip
Floorplate
So here is the parts list you need to go by for your SKS...
1. Receiver
2. Barrel
3. Muzzle attachment
4. Bolt
5. Bolt carrier
6. Gas piston
7. Trigger housing
8. Trigger
9. Hammer
10. Sear
11. Disconnector
12. Buttstock
13. Forearm/handguard
14. Magazine body
15. Follower
So this brings us down to 15 BATFE recognized imported parts and if you change anything about your SKS, you need to get that number down to 10 or less. Adding a domestic magazine will get you two parts (the follower and the mag body), Adding a new stock will get you 2 parts (the butt stock and the forearm), removing the grenade launcher and/or replacing it with a muzzle brake will get rid of another imported part thus getting you down to 10. There are other parts you can replace with American parts too, it just depends on what you want to keep and what you want to replace.