I appreciate your "let's not destroy working kit" disclaimer, but I'm thinking in terms of fabrication, I wonder if you'd have an easier time "cladding" a conventional ATX case with a custom bezel that gave it a PS/2-inspired look, perhaps 3-D printed or assembled from sheet plastic and sandblasted to give the right finish, and given a beige paint job.
If nothing else, fitment will be a nightmare on a real PS/2:
* The Model 60/80 is super-narrow by modern standards-- just 166mm. For comparison, the new ATX tower I'm using is 212mm wide. That's going to severely limit your choices on cooling and larger GPUs. You might not even be able to do a rear-mounted 120mm exhaust fan, or a closed-loop cooler, without big surgery.
* If you want to have optical drives, especially mounted "horizontally" like most PCs, I doubt they'd fit. Design-wise, they're sort of late to the PS/2 era-- I'm not sure there's a proper "standard bezel" for them the way that PS/2 floppy drives have a distinct look. You'd likely have to invent something from whole cloth.
One interesting option is to use a momentary on DPDT switch and set it up so the paddle sits in a "central" position, and pushing one direction closes power, and the other closes the reset switch.
