^^^
What nglevin said. Just install an ATX PS.
I replaced the PS in my MDD G4 twice and was ready to scrap it when the stock PS failed for the third time.
However, I decided to try the ATX path and it worked like a charm!
Follow the instructions here:
http://atxg4.com/mdd.html
I didn't use a mod cable I just rearranged the pins using the proper molex extraction tool. (Without which it's basically impossible to remove the pins.)
I did have to use an 24 pin extension cable to be able to mount the PS inside the top of the case.
Also, I haven't had any issues with "deep sleep" although I usually power it off when I'm not using it.