Varies some by motor, but generally as the motor gets to around or over about 60C / 140F it likely will fade in power some.

Generally above about 70-75C / 160-170F it starts to demagnetize the rotor. That reduces power even when cooled back off and makes the motor run hotter. Most motors have replaceable rotors.

Hotter risks burning up the motor.

See if some of the guys will help you with an idea of the gearing that works for the class and track for a starting point.