You ought to be able to do that. The only two things you're missing would be the remote control box and the Video/power switch board, but those ought not be too hard to come by, even locally. You should be able to test it without those parts, especially so if you have or can borrow a PCI video card. It might be would be worth getting a 6xxx Road Apple for the extra parts. The video board is probably simple enough to build from what I recall.

ATX PSU and case is no problem: Performa 6500 goes digital . . . equipment corp.