I also happen to have a 1.25GHz MDD with a failed PSU and i am now using the atx psu of a broken Maxdata desktop PC.
I tried analysing the defective psu but neither rectifier, capacitor or anything else on the primary seems broken, the breaker kicks out as soon as i connect it to my wall outlet. What ponders me though is that the integrated fuse will not blow, though it is rated much less than the breaker.
Be careful when using a new atx power supply! the connector will fit but the pinouts are different and you my end up frying either your logic board or the new power supply. if you don't wanna void the warranty of the new power supply, buy an adapter for 20->24 pin ATX and tinker with that to match the pinout. Use the broken psu for reference as the wire colours will match in most cases. if in doubt, use the pinouts on pinouts.ru or wikipedia as reference. Unlike the ATX standard the MDD logic board does not have a pin for PWR_OK and -5V, so you can leave them unconnected.
Also note that the psu needs to have a really strong 12V line (10A+ recommended) and it can not be any higher than 7cm to fit inside the MDDs case.