The only place you are going to find another rear window motor is from another Travelall. I'm pretty sure they are not available unless you adapt something and make it custom out of a similar sized power window motor like from an old station wagon.
I swapped one from a 73 into my 72. The motor design and wiring were different but close enough that I was able to modify the wiring and make it work like new.
See if the motor itself is damaged, if it is not visibly broken or missing, it's a motor, it can be rebuilt.
Also, check the switch, fuses, relay, and measure the voltage at the motor connector while operating the switch. Don't forget about the little switch in the tailgate, it disables the motor unless the tailgate is closed.