Karen when I first started on my Tom Thumb I was going to use a 49cc engine.I found a complete pocket bike for $50 and took it apart including the motor mount plate and it fit in perfectly.I would have to customize the exhaust but it could be made to fit. My only problem was in figuring out the gearing using a jackshaft. In the end I chose the West Bend 820 which I already had and I could get it to fit. I agree the O&R won't give you enough power. The other option is a 2 stroke Tecumseh motor. These engines as with all 2 strokes they can be run on their side which with only 8 in clearance this certainly helps.There is a pic of this set up under the OldMiniBikes past articles when you type in Tom Thumb. Also the 49cc motors run small chain so you will need there sprocket as well. If I can help further--be happy to and your bike looks great--Bill.:thumbsup: