You can legally swap your thumbhole stock on your Mak90 with having to swap out your Chinese parts for US made parts by upgrading to the ATI Fiberforce stock Advanced Technology Black Glass Filled Nylon, Thumbhole AK-47/MAK-9o. See this list for other ATI stocks such as Mini 14, Remington 870, Ruger 10/22 ect.
Advanced Technology Rifle Stocks are a great way to upgrade or improve your shody surplus military guns. I’ve used the ATI Stocks on my Mak90 gun for years because the awful thumbhole stock it came with really ruined the image. The absurd gun laws that were implemented in the early 1990s really dumbed down the looks of some of the surplus AKs that came into the Country.