Gigadesigns had an upgrade for the MDD, about a year ago. But they don't sell it anymore, I guess there wasn't much demand.
What you can do is try to find an original Apple Dual 1.42GHz module, on eBay for example. In that case, you can simply swap the CPU modules, and no problems with firmware, recognition, or whatsoever (can sometimes happen with 3rd party upgrades).
But make sure it comes with it's original heatsink, as it is different from the Dual 1.25 module.
You could buy such a module from Apple, through an Apple repair center, but it's not worth it, as the module alone (without heatsink) costs about as much as a Mac mini Dual Core.