Motorcycles are for rent everywhere on the island. However, accidents are VERY common. I saw a girl arrive on Koh Chang, rent a scooter, and crash within 2 minutes. I go to Koh Chang about once every 3-weeks. I have never been there and not witnessed someone limping around. I've witnessed crashes ranging from scrapes, bumps, and stitches, to compound fractures with the bone sticking out of the leg.
By all means, rent a motorcycle. But I can't stress this point enough: don't be overconfident.