Actually, the SKS is not considered an "assault weapon". You will be altering it out of the C&R status, but unless you add a seperate pistol grip to it you should be OK adding a high cap magazine to it......for all the good it will do you. I've never seen a hi-cap work correctly in an SKS of any type. The bayonet can stay as the no-bayonet thing is only for Chinese guns to meet "sporting purposes".
Really, there is no reason to be adding one of those crappy "Hi-cap" mags to your SKS unless you really have a serious need to torture yourself. They suck. Get some strippers and practice reloading, it's surprising how fast they work once you get the hang of it. The SKS is *utterly* reliable with it's fixed ten rounder; it is guaranteed to be a jam-o-matic with a crappy "Hi-cap". I love my SKS's, and long ago gave up on all the aftermarket stuff made for them- 90 percent of it is crap. I keep my SKS's for the history and workmanship they exhibit, and my AK's for the big mags, seperate PG, and general servicability. Your SKS just isn't an AK, and I personally recommend against trying to make it one.
Still, that's just my opinion.