I recently replaced my opener in a similar situation. Mine's chain drive and I did reuse the existing chain. I'd be less confident reusing a belt; rubber degrades faster than metal. But, based on my experience, replacing just the belt or chain isn't much harder than replacing the motor unit. I had to loosen my chain considerably in order to get it around the new motors sprocket. At that point, it would have been just a few more turns of the tension screw to fully remove it.