If you update the stock you will have to update other things as well to make it compliant. Mine grew on me, although I did strip an stain it a better darker color.
I had a Mak90 about 3 years ago that had the thumbhole stock on it. I didnt like it at first and looked into changing it up a bit but never did. In my opinion, its a great, fun and reliable gun, there is just no need to really try to make it anything more. Although the new stocks look really cool on them. Sorry more comments than help.
I have two maks I actually think the thumbhole stock feels good. The only problem I have with it is the added weight. My other aks definintly feel lighter with a folding tapco stock and the other has a folding wire stock the weight is the main advantage of the other stocks. I hate the wire stock though, it just does not feel good or shoulder properly and would not be recommended.
The problem arises from a very stupid law that states if you modify the rifle as shipped, you have to replace enough foreign parts with US parts so that there are no more than 10 imported parts in your rifle, otherwise it is considered a semi-automatic assault rifle. With the rifle as delivered, or with 10 or less imported parts it is an angelic sporting rifle incapable of hurting babies.
Known as 922(r) or Title 27 CFR478.39, a rule that violates every bit of common sense known to mankind.
I believe it makes a difference if your Mak has the straight cut or slant cut receiver, plus like others said you have to update with U.S parts to be compliant. I recently picked up a Mak-90 too. I actually like the thumbhole stock, the way it feels, and the look of it.