I managed to shim the v-belt pulley and jerry-rig a motor mount, so I mounted up the little motor onto my Piaggio Boxer scooter, put the v-belt on, strapped the batteries on with duct tape, and shorted the throttle wires, and it ran!
The speed was horrendous, maybe 6 mph on a slight downhill and 3-4 mph on a level surface. But that's fine for the purpose of my test run.
Well, I went to Tractor Supply Center (TSC) today and all they had were imperial sized pulleys (1/2" and 3/4" bores). The motor that I have must be a metric size one because the 1/2" hub pulleys were too large. I bought some 1/2" hub pulleys anyway since I wanted to see if it would work as a proof of concept (and just around how much more power I'll need).
Now I just have to make a mount for the motor. I think I'll just use a piece of 2x6 wood that I have laying around and make a simple mount.