I feel like a broken record:
OFNA Spur Gear Question
That link I posted is the most economical part that is likely compatible from the same OEM
According to this manual here for the Ultra MBX R2 they specify the stock spur as #31040
https://www.manualslib.com/manual/76...?page=6#manual
https://www.ebay.com/itm/175835724837
I've ran enough HongNor cars that chances are you may get a full diff to work as well if you measure the bearings and distance between the slots in the diff mount then ask someone on RC Tech to measure and confirm:
https://www.ebay.com/itm/255127759402
After you take your measurement (outer bearing to outer bearing) and verify bearing diameter, then ask someone on this thread to confirm measurements:
https://www.rctech.net/forum/nitro-o...in-x1-crt.html