Had an old PowerPC Mac case a while back, and made a few changes to mount ATX hardware.

The front plate for the previous I/O was completely cut out, leaving a rectangular cut in the case. This cut was bridged by an adaptive bracket, allowing for the mounting of an ATX I/O panel.

The rest of the build featured a rewired switch (using the original but allowing it to work on the ATX motherboard) and mounting the PSU at the bottom of the case.