I think I made a mistake, user Chubaka is right- all the links I posted are for shaft-driven vehicles, #A912 was for the RS4 MT series - I don't know that any of them would be usable in a belt-drive vehicle, at the very least it wouldn't be drop in. If the pics you posted are of the car you're...
I found this listing on aliexpress, it's a little pricey but the vendor looks reputable.
The redcat 40/37T might work - BS903-103, looks like an almost exact match. Ebay link
You might also be able to drop in the transmission from an HSP flying fish, it's nearly identical - the whole car was...