All 870 parts will fit. Get the 870 mag cap made for the black plastic spring/mag cap retainer. It has ratchets at the top inside the cap that mate up with the ratchets on the spring retainer.
The Hawk 98 shotguns have the smaller M3 ghost ring sight elevation set screw. It can loosen under recoil and the ghost ring aperture can slide off and get lost. Use a very small amount blue loctite to keep it tight. The newer Hawk 982 has the larger M4 elevation set screw.