I would just go buy a new logic board for the G5 and call it a done deal.

As for the power supply on the MDD you can buy a cheap-o ATX power supply and repin it. For the special 24volt lead you can use ether a power regulator or use a A/C to D/C wall adapter.

If you're low on funds you can merge the two systems together like using the power supply from the G5 and maybe buy a second hand PCI SATA card and use the hard drive/s as well.