I'm about to trade for a MAK-90 that has a thumbhole stock on it, but will come with a welded open underfolder and a pistol grip. Are these legal to install? Please advise the LEGAL way for me to be able to do this.

See if you can cut a deal without the welded open underfolder and pistol grip! (unless the MAK in question already has the underfolder trunnion and accompanying holes in the receiver)

To a degree, it could be done legally. To install a pistol grip and pre-ban style stock set, the MAK90 needs to have five US parts installed. These can be accomplished with any combination of the following parts: the fire control group (counts as 3),Stock set (counts as 3), Gas piston (counts as 1), magazine followers/floorplates (counts as 2).

Easiest would be the fcg and stock set, giving you a US parts count of 6, thus allowing for a muzzle brake as well.

The problem with the welded open underfolder is the MAK90 receiver. Surf the web and check some pics, the underfolder receiver is different at the rear and uses a different trunnion. You could cut the rear tang off of the receiver and drill the receiver for the stock but it would take a great deal of work to come off with a proffessional appearance.

If the rifle was an underfolder prior to being "sporterized". Use a US pistol grip(1), forends(1), swap the fcg(3) and you're good to go.

LIST OF PARTS FOR YOUR POSTBAN
THAT QUALIFY AS USA MADE

§ 178.39 Assembly of semiautomatic rifles or shotguns.

(a) No person shall assemble a semiautomatic rifle or any shotgun using more than 10 of the imported parts listed in paragraph (c) of this section if the assembled firearm is prohibited from importation under section 925(d)(3) as not being particularly suitable for or readily adaptable to sporting purposes.

(c) For purposes of this section, the term "imported parts" are:

(1) Frames, receivers, receiver castings, forgings or stampings
(2) Barrels
(3) Barrel extensions
(4) Mounting blocks (trunions)
(5) Muzzle attachments
(6) Bolts
(7) Bolt carriers
(8) Operating rods
(9) Gas pistons
(10) Trigger housings
(11) Triggers
(12) Hammers
(13) Sears
(14) Disconnectors
(15) Buttstocks
(16) Pistol grips
(17) Forearms, handguards
(18) Magazine bodies
(19) Followers
(20) Floorplates