Try to look for a Makerspace in your neighborhood
if you ask nicely they may help you out.
"sir sounds like the transmission, if you like we can order you a new one, the rebuild never works, the other side starts to go too, you should get a set, go to the office pay a none refund deposit, when the parts come in from China, we will give you a call. Ok Sir."
if it's not sentimental to you put it some ware they can't find, by Christmas time the kid will want other new toys anyway.