Here's how to fix a transmission gear from an Axial SCX10 crawler.
In this video you will see the components of the transmission and how to disassemble and assemble the transmission.
The original transmission operated for about a year. This car has been driven in deep mud, cold conditions in winter and steep ascents and descents. Suddenly there was a rattling sound from the transmission and the car stopped running. When I opened the transmission I noticed that one of the gears had worn badly.