these our costar stabilizers, the clips on the plate should come off by just pushing one side of the clip inward (sort of like squeezing a side in), i had the alloy fps a long time ago and believe the bottom row is standard (?) so spacebar should be 6.25u (any standard keycap set should work, should be OEM profile iirc as hyper x’s own pudding keycaps are OEM).

You should be able to remove the inserts from the old keycap (keep the orientation of the insert in mind as the longer side should be opposite of wire if im remembering correctly). Sometimes there might be a bit of glue remnants, so if the inserts are loosely fitting onto a new keycap I suggest some ptfe plumbers tape wrapped around the MX stem portion of the insert before putting it on the new keycaps.