One year ago my IIci PSU got broken, I don't care about repairing old PSUs so I modified an ATX in order to use it on my Macs.
I did some diagrams and a video to explain how to do this (audio is in Italian but I added english subtitles)
1) First I broke a 20 pin ATX male plug, the left part fits good in the 10 Pin PSU input.
2) Look to numbers under the PSU input on your Macintosh motherboard.
3) Follow the diagram I created, remove 3x GNDs, 3x +5V, 1x +12V, 1x -12V, 1x +5V_SB from the 20 Pin ATX male plug then put them into the adaptor you broke following my diagram (assure yourself that fits correctly into your motherboard)
4) Put a rocker switch between PS_ON (Green cable) and a GND (a black cable) for turning On and Off your Macintosh (you must shut it down it after doing Special > Shutdown)
LINK TO MY YOUTUBE VIDEO: