Which bits are damaged?? The plastic gears??
If they are you must make sure you " shim " the diff up against the bevel gear. You need to get the very thin spacers (shims) put these onto the outdrives (before the bearings) place into the gearbox casing then check the " backlash " (put shims on either side until you feel its ok)while holding one gear so it doesnt move, rock the other, you need a small bit of movement. Once you have this right, put some teflon grease to lubricte them.
When checking all this remove the main blue drive shaft this way you have a better idea on the adjustment you have made.
When you add the grease you can spin the outdrive ( the bit the blue center shaft goes into ) with your fingers to get the gears coated nicely. DO NOT put an excessive amount of grease on, just enough to coat the gears otherwise you may with ise contaminate the diff!
If you need info on diff building, let me know
