Will a Apple PowerMac G4 MDD Dual 1.42Ghz Processor CPU work in a Fire Wire 400 MDD?

It should work. I think there's an article over on xlr8yourmac.com (click "Systems" in the title bar and then "G4" for the relevant articles.

If you have a 1.25 GHz card, it can probably be overclocked to 1.42 GHz by moving a few resistors, so no need for a 1.42 GHz module.

If the FW400 MDD is one of the models with the 133 MHz bus, then you'll need to do the bus speed modification to 167 MHz first, if you want the card to run at 1.42 GHz. Otherwise it will run at 1.13 GHz.

If you must do the bus speed modification, then you must also replace your DDR RAM unless it is already rated for the higher speed.