Dual 1.33 GHz G4 CPU cards are also commonly available for the MDD. They come from the G4 Xserver. They're easier to find, usually, than the dual 1.42 GHz cards.
One Ebay seller has had new ones (Apple Service Parts) for sale for $75 + shipping for a couple of years. But it doesn't include a heat sink, which could be an issue if you're going from single CPU to dual CPU.
I've often toyed with the idea of trying to mount some of the LGA775 heat sinks in the MDD on the dual G4s. The trick is to find one with a long enough base to cover both CPUs (some have it) and which isn't too tall to fit in the space. Also, oriented so that the fan is blowing through the fins and not against the fins when the heat sink is covering both CPUs would be good.
Of course the other trick would be to machine some kind of base to mount on the MDD screw holes and provide the proper mounting holes for the LGA775 heat sink.