There are no internal parts on the MAK90 that can be improved so it makes not sense to buy any of that stuff.
I have compared a Polytech stamped receiver AK alongside my DCI/Polish parts AK with Tapco fire control parts. While I'd agree that I like the overall look, feel and finish of the Chinese AK parts over the Tapco parts, the Tapco fire control parts feel much better in use than the Polytech parts. Basically, there is far less take up and overtravel with the Tapco parts, and the trigger pull is much cleaner - nearly that of a decent AR. Part of the take-up is eliminated because the receiver has a little protrusion in front of the trigger that acts as a trigger stop - eliminating a lot of the take-up.