I remembered about several types of PSU form factors smaller than the typical ATX, and FlexATX is the closest to the original MDD PSU, albeit still without the same mounting holes and possibly with even smaller 40mm fans. However, I think those would at least fit where the original would have been, with some clever mounting work.
This would also be a prime opportunity to slip in a modern 80+ certified PSU design, hopefully dumping less waste heat into the MDD case. However, 300-400W PSUs in FlexATX form factor don't come cheap, roughly $60-90 for the good Seasonic and FSP units! Also, you'll have to do something about all the SATA power connectors instead of old-fashioned 4-pin Molex, which would be considerably easier if it's a modular unit (just depin the modular cables with a Molex extractor tool and connect new ones, rewiring the main power cable to MDD spec in the process).
There is one other big concern, and it's that small PSU fans mean LOTS of noise, as the original PSU proved. I haven't seen any FlexATX PSUs that opted for a larger fan on the side, so I'd probably have to whip up a larger fan duct like the one in the stickied MDD silencing effort thread.
Still, as original MDD PSUs continue to die out, this seems like the best method going forward if you don't want to give up your entire optical drive bay to an ATX PSU and don't need ADC power, at least until someone's willing to design a new PSU from the ground up just for the MDD.
Has anyone else put a FlexATX PSU in the MDD before and mounted it without any hassle, perhaps with an ideal PSU model to suggest for the rest of us?