Because the muzzle on the rifle is threaded, you need six compliance parts instead of five to meet the 922R requirements.
If the lower handguard you want to keep is part of the Ultimak mount and both are U.S. made, then the combo will count as one compliance part. If either the lower handguard or the upper handguard is foreign made, then there is no compliance.
You will be switching a foreign made stock for a U.S. made stock so there is no problem as there is no restriction on the number of U.S. parts you can add to the rifle. The U.S. made stock will count as one compliance part.
Changing the FCG for a complete U.S. set will give you three compliance parts. The piston may already be U.S. made, check with the manufacturer to find out - if so, then you will have 5 parts. (Century International uses U.S. made gas pistons and some of the others may also.)
A U.S. muzzle device will give you 6 parts.
Let's recount to be sure:
U.S. buttstock - 1 compliance part
U.S. FCG (complete set) - 3 compliance parts
U.S. gas piston - 1 compliance part
U.S. muzzle device - 1 compliance part
If you learn that the gas piston is not U.S. made, install a U.S. made pistol grip and you will be in compliance with 6 parts.