Whether or not the 24 Volt motor gets damaged when running it on 36 Volt depends entirely on how hot it gets because it is the heat that damages motors and not the Voltage. If it is used gently and not at full throttle continuously then the motor may survive being overvolted, however, if it is used hard and ran at full throttle continuously or used to go up hills or tow other people then it would most likely overheat and burn out fairly quickly.
We have not installed a Crazy Cart XL motor onto a Crazy Cart before so we do not have any tips or pointers on how to do that yet.
Gavin Costigan