I would imagine a regular ATX motherboard will fit, this is a good idea actually given how unlikely I am to find an Amiga 2000 motherboard and known good PSU to use with my two empty cases and stack of ICs pulled from WB2 machines.
Only question I have is, can I use my original A2000 keyboard on a PC motherboard without problems? The A2000 keyboards I have are really nice to use. Obviously I need a PS2/DIN adaptor but other than that is it OK to use?