The odd thing, I find, is that I can't get the gearbox on their own. I guess it's probably more economic to simply get the whole assembly premade than it is to have the parts shipped in separately. If you do end up replacing the gearbox though, you will want a like for like replacement as so to match the ratios on the other side.

However, if the gearbox is bust because of a nylon gear breaking, it makes longer term economic sense to get a pair of motors with planetary gearboxes on them which are all metal, like the Gimson range.