another thought, maybe your main bearings are shot. if so it might cause the difficulty in pedaling and the engine may not produce enough power to turn the crankshaft causing it to die.
taking off the "drive belt" just means the chain that drives the wheel. you want to try to move the engine only. not the engine and the back wheel. if you take off the chain and it moves freely, then the problem is with the wheel. if it doesn't then the problem is in the motor.