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Diff is standard, but to fragile for the monster wheels. Kontemax an TC has some nice posts regarding monter racer gearboxes. The car came with spare diff parts and was in good shape except that the gearbox plastic got brittle and some parts needed replacing. Haven't finished the new shell yet. So project is still open.

I don't remind which type of diff is in a madcap but it could be a possibility. other way, look for an avante one. they lookvery similar. good luck for your resto.
ps Waterbok, your racer is tooooooo nice