MAK90 is a solid gun if you can find one.
It has a chrome barrel and bolt/op rod. Does not corrode and I have shot corrosive ammo in mine and not an issue. Most solid AK out there bar none.
Keep in mind if it has been changed from Thumb-hole to a conventional stock it has legal issues. Easy to deal with but a possible BATF problem
Basically it can only be changed from Thumb-hole to conventional if it has 5 US made parts (an updated Thumb-hole I think is ok but have not looked at that).
Easiest is having a US made stock (3 parts, not all the individual parts like upper hand guard count) and a US made magazine (3 parts which means the spring does not count). Gets you over the top.
There are no internal parts on the MAK90 that can be improved so it makes not sense to buy any of that stuff.
Severl US made magazines if you want multiple at the ragne. You can keep your old ones, just not have them in the gun (i.e backups only if you get into a sitation no one cares, i.e. SHTF.