As the risk of raising the ire of armchair firearm law legal beagles, there is no such animal as a "pre-ban" MAK90. A MAK90 would not need the thumbhole stock for compliance if it were preban! Mak90s are postban.
That said, you CAN convert a MAK90 legally to pistol grip configuration with a US made pistol grip and the three fire control group parts to get the proper US parts count, and you can keep the existing handguards. OR, install a US made stockset for 3 parts, and a US hammer or trigger for the fourth part. I converted a MAK90 to pistol grip configuration using some maple dimension lumber I had in the shop and fashioned a stock and handguards with it, and used a cheap black US-made plastic SAR pistol grip off the trader, and added a Gordon Technologies double-hook FCG.
As far as the $$$ on the weapon and magazines, MAK90s go for $350 here too, the drum for maybe $75 to $120 depending on who's asking, and the 30s run about $8 to $15, again depending on who's asking. As priced at $500, no great deal. IMO, $350 is too much to pay for a MAK90 and then spend another $125 to $150 for a US stock set and FCG just to make compliance. If I were not able to make my own stockset and document it in digital pix, I would not have bought the $325 MAK90 that I did. All told, I don't have $360 in it.
Again, NO such thing as a preban MAK90. There were some post 89 and pre 94 Chinese AKs that were converted to butthole stock configuration after the 94 thing went into effect, but those usually have muzzle threads and are the exception.
Noah