Unfortunately, at least to my understanding, if you modify an SKS out of its original configuration you will fall out of compliance with 18 U.S.C. ยง 922(r) and 27 CFR 478.39. If you change out of that configuration and add an aftermarket muzzle brake/fash hider/ barrel attachment you must then change out enough "regulated parts" to have no more than 10 foreign made parts of an imported semiautomatic rifle. On the one that I modified I changed out the following bolded items out of this regulated parts list. I then tallied the total number of foreign parts and made sure that there were no more than 10.
27 C.F.R. 478.89 lists 20 parts:
(1) Frames, receivers, receiver castings, forgings or stampings - part tally = 1
(2) Barrels - part tally = 2
(3) Barrel extensions (not counted on an SKS)
(4) Mounting blocks (trunions) (not counted on an SKS)
(5) Muzzle attachments - part tally = 3
(6) Bolts - part tally = 4
(7) Bolt carriers - part tally = 5
(8 ) Operating rods (not counted on an SKS)
(9) Gas pistons(10) Trigger housings - part tally = 6
(11) Triggers - part tally = 7
(12) Hammers - part tally = 8
(13) Sears - part tally = 9
(14) Disconnectors - part tally = 10
(15) Butt stocks
(16) Pistol grips
(17) Forearms, hand guards
(18) Magazine bodies
(19) Followers
(20) Floorplates
Good info can be found here
http://www.tapco.com/section922r/ from Tapco, who makes parts for compliance. Though, it seems their lawyers have nudged them away from offering total complete compliance packages anymore.
Enjoy that SKS! I'm actually looking at selling a pristine factory Yugo M59/66A1, but will require a lot more than $80 to part with it.