just bought a mak-90 with thumbhole stock..If I want to convert it to regular,pistol grip,buttstock..Is there anything else involved.Do I have to change parts to us parts or is it just a simple change..Just curious ..Thanks

you'd need 5 US made parts to be 922r legal.  the easy options are stock, pistol grip and handguards (both handguards count as one) along with magazine followers and floorplates.  you can also replace the fire control group for 3 parts (trigger, disconnector and hammer).

if you thread the muzzle and add a brake you'll need 6 US parts total, so most go with the stockset and FCG.  if the receiver is angle cut ironwooddesigns.com has some nice stocksets that fit.