If you want to replace the stock on a MAK-90, it's simple enough, but you will need to ensure 922r compliance. In short, non-sporting firearms need to have 10 or fewer imported parts. With the thumbhole stock the MAK is sporting, but not once you go to a more standark pistol grip and stock (folding or not).
On the MAK-90, you need 5 US made parts. If you use a US made stock and pistol grip, that leaves you with 3 more parts needed. While you could restrict yourself to US made magazines (3 parts), the moment you put an imported mag on there you'd be out of compliance. I'd recommend swapping out the trigger group (3 parts)- it's quick and easy (get the retaining plate that replaces the shepherd's crook spring while you're at it; reassembly is easier).
So long as you install 5 US made parts and ensure you have the right stock (some MAKs were milled receivers, some were stamped, and the stock fit is different), you're good to go.