You'll need to replace certain parts with U.S. made parts. Most likely, you're looking at the very least the piston, the hammer, trigger, disconnector and something else.
My mind is rusty on the matter.
There are a TON of threads an information if you do a quick search that cite the actual law, the actual parts lists etc...
I am very confident that the MAK-90 cannot be altered to have a folder legally unless it is converted.
They are no different than Saiga rifles. Any foreign semi automatic magazine fed rifle cannot have certain "features" otherwise it is an assult rifle. In order to have these features, you have to make it into a domestically made rifle, and the definition of that is the proportion of U.S. made parts vs. Foreign made parts.
It's really stupid, but it's the law. RESEARCH before do you anything!