Well, looks like I'll take one last stab at this. I've ordered the motor I ORIGINALLY INTENDED TO USE. A MY1020 rated for 60-volts.
TNC Scooters (Scooter Sales and Service)
The only problem I had with the first motor was that it wasn't able to handle the higher RPMs that resulted from over-volting. It never had any issues with the loads placed on it otherwise.
I also have a plan for mounting it so that I can get rid of the belt tensioner.