I thought about upgrading my rear wheel but having the same luck as you. Allot of bicycle rims sometimes aren't up to the task of holding up on a motorized bike. Finding good rims for a good price is tough for me to. I was thinking of trying to find a good upgraded 24 rim for my rear wheel because my bike has a 24 in the rear & a 26 in the front because it has a fatter chopper style tire in the rear. 24's are harder to find than most rim sizes.
I've heard the horror stories of motorized bikes & bent rims so upgrading or replacing rims is almost unavoidable if you wanna go fast. Wish someone made moped wheels specifically for motorized bikes.