LOL they are fun to ride around. You could find engine kits on the internet. If you decide to buy one I would suggest the 4 stroke 49cc. These are more expensive but much easier for beginners to hop on and ride being that it is an automatic. Also the 4 stroke engines would be more reliable as they are regularly used and tested in industry work on water pumps, tillers, mowers etc. In most areas under 50cc is considered a moped which does not require a tag, registration, or insurance.
This bike has the 4 stroke Honda GXH50 with a gear reduction box and centrifugal clutch. It was more complicated to install than the 2 stroke kit but well worth it in my opinion. I spent hours fitting, welding, and fabricating to make it work right. The motor mount plate didn't fit the bike frame and needed modification to work. I had to cut the exhaust apart and reweld it to clear the frame. The original pedals hit the frame so they were cut off and welded new extended pedals to the original crank assembly. Be sure to check the knot for the pull start handle because mine pulled off right away. This caused the string to go inside the recoil and I had a little adventure fixing the recoil.
Once the bugs are worked out they are a blast. I've driven these bikes to school and everybody loves them. I'll see random people waving and smiling all the time on these things. I planned on having the monster limo completed by the "senior day parade" at my high school in two weeks, but got behind with work. The other idea was to ride the Silver Super Swamper cart, now that its sold I am down to the red Schwinn motorized bicycle...which will be fun too.