Hey goat.
I wonder if you might have two problems causing the wheel spin.
First is the drive tire diameter.
I would guess you would have to gear it to the moon with the small rear tires to keep it from being under driven.
Then, what about rotating mass of the fly wheel.
I believe the fly wheel for the 87 is a lot lighter then it is for the newer 500's.
Can you tell me the sprocket sizes front and rear are, as well as the rear tire diameter?