Just keep the fixed mag.
Here is my understanding. First, forget about federal law. You will break the state law (SB 23) before you break the federal law. My understanding is that you cannot have an "assault rifle" without a detachable magazine. As long as you keep the standard fixed magazine, you are not going to break the law. Should you put a detachable magazine on your SKS, then you are restricted in the following ways.
You may not have:
1) a bayonet.
2) a pistol grip stock.
3) a folding stock.
4) a thumbhole stock.
5) a flash suppressor.
And there might be a few more no-nos, but those are the main ones that apply to the SKS.
I bought a fixed folding pistol grip stock for mine a few years back. I didn't like it because the stock had about an 1/8" movement where it was fixed and the pistol grip and the trigger guard didn't match up the way I liked. Also the bayonet came out away from the stock so if my hand were to slide forward, I would basically wedge my hand between the bayonet and the stock! When I sold one of my SKS's to a friend, I added the stock in with it and just put the original wood back on my Russian SKS.
I would ask yourself, what are the benefits of replacing your wood stock? I tried it and found that it was best to just leave it as is. 2nd benefit, there is no mistaking the law because it is in its orginal condition. Whatever you decide, good luck.