Norinco has unique stock fit needs.
Also some are slanted and some straight on the back of the receiver.
Other stocks can be made to fit but takes some wood work (not sure on synthetic but suspect the same)
I think Ironwood is one of the few that makes a wooden conversion stock for the MAK 90 though you do have to tell them what the cut is on the the badk.
If you do change stocks keep in mind that you need to have 5 US made parts in or on the gun to make legal.
Easiest is to swap the stock and then get a US made magazine (if you can) so use as a magazine lists as 3 legal parts (gets you up to 6 ad standard AK stock is only 3 as well, the upper hand guard does not count.
Other internal parts are available but they add nothing to the MAK as it came with a chrome bolt and the best trigger you can get on an AK