I will second the livefast suggestion. They have a package to mount to the Stingray. So you'd need to get a motor kit, then the stingray kit.

http://www.livefastmotors.com/packages.html

Also, I'd watch out. A lot of states require that you have a drivers license to ride a motorized bike. Might be helpful to check your local laws, and maybe even stick with the 50cc for a little more legal safety.