Lots of places to rent a bike or scooter in Negril. It's always good to make note of scratches, tire condition, etc., on the rental contract to avoid misunderstandings later. Don't be afraid to negotiate on price if you are renting for a few days or more. You probably get a better deal if you go back to the same guy year after year too, say you've rented from him before, it's a good deal for both of you.
Gas is a good choice, I have rented from him several times, as I have with Prento.
Gas has moved across the street to the seaside beside Blue Cave Castle.
Drive safely, enjoy the hills!