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.
I guess my question was whether the stats, e.g. number of teeth per inch, width, thickness on garage door belts are standardized/universal. It’s good information that you were able to reuse the chain and replacing it isn’t that hard to do, thanks!