I've occasionally seen a joblot of "spares or repairs" drills (argos and others) appear on eBay. As Lucy says though, you need to make sure you have the same type all round or you'll go in circles.

If those are the three gears right after the motor that have broken, I would guess it's not down to the mounting. If it was the last set of gears before the output shaft that had died, or the output shaft bearing even, that would look like an impact on the wheel. I suppose maybe the motor slipped out of the gearbox (that happened to us several times at Robodojo), and then it span enough partially meshed to damage the gears?