No info on scooter rentals, but I have rented a car on St. Maarteen. Before the guy handed me the keys, he hollered to his helper, "Is this the one with the good brakes?" , I didn't have any problems, but this "chicken" will be "stickin" to transportation either 4 wheeled or 2 legged variety. You might try the usual rental car companies. If they don't rent scooters, maybe they can direct you.
We rented a scooter in St. Martin two years ago. There were two rental paces about a 10 minute walk from the dock. Compared to some of the other islands, this was a good place for a scooter, as the roads are relatively flat, in good condition and well marked. We had a wonderful day, but I would only recommend it to people experienced on a motorcycle.