Converting a Bicycle to a Motorbike

This chap can!

Try Google for power assist bicycle!

Results 1 - 10 of about 1,530,000 for bicycle power assist. (0.33 seconds

try to get a Solex!

Just replace your front wheel with a petrol engine:

http://www.treehugger.com/files/2006/05/the_wheel_turns.php

Here is a nice little bolt-on kit. And it's worth a look at the photo gallery.

http://www.zbox.com.au/

This may not be a good idea, based on my experience of the Vincent Firefly, a conversion made in the 1950s. It punctured tyres regularly due to extra weight, the brakes were no longer adequate (especially in the wet), and the vibration levels were unpleasant to the rider and caused every part that could come loose to do so. I suggest the engineers that design small motorcycles (AKA mopeds) do know what they are doing, and they do not use pedal bicycle parts for a good reason.

I used to know a guy, who at five years motorized his tricycle with a lawnmower engine.

The five year old did, not his dad. When the guy was three he used to bring his dad's tools to him while working on engines before he asked for them. The kid was a genius.

IF I were interested in this conversion I would check out small chain saws, particularly with regard to the rpm at max torque and the pitch of the chain. If you can find a match between the pitch and your bicycle's chain pitch and the Horsepower/rpm match your target speed then all you have to design/build is teh engine mount as most chain saws come with a centrifugal clutch. Check out the load rating of your tires. I would suggest that the 2.125" wide ones would be more satisfactory than the 1.75 or 1.375 sizes.