My Optima runner has been converted to belt drive by just replacing the belt and diff cases with Mid parts. Works fine and reliability hasn't suffered because the idler gear isn't in there.
If you want to do a proper full conversion there's absolutely no need to search out what is just the standard gearbox that someone at Kyosho cut a section away then drilled two holes. If you've got the lexan cover you even have a cutting and drilling guide, or just copy the photo above
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