Tamborista,
We rented a scooter in Sosua with my GF on July 3, drove around for about three hours (Sosua is not that big) and then about an hour after lunch, both of us still alive. There are a few places in Sosua where you can rent motorcycle, scooter, or 4 wheeler, just ask in your hotel or about anybody on the street. Rent from the place that actually has an office and established parking lot (some muchacho was trying to "rent" us a motorcycle parked by the tree, LOL). You will need to give your passport or ID as a deposit, and agree to pay for the damages if scooter is hit. I think helmets are mandatory for motorcyclists, but not sure if this applies to scooter riders. AMET will take your license for traffic violations, even if you are a tourist. Have fun and be VERY careful.