I'm currently working on my Rack mounted 2.5 Harbor Freight Greyhound engine.

I'm mounting in my shwinn del mar beach cruiser.

Everything is custom made.

For this build you need the 2.5 engine. ( pick it up  at a Harbor Freight store for 99 bucks )

Then you need a 5/8" centrifuegal clutch.

Then you will probally need a jackshaft. I had to make one. ( It's for gear reduction )

Your gonna have to buy a good bike rack made for the style of your bike and reinforce it or make one completly from scratch.

Then get some go kart chain size #40/41. Make sure your gearings right. Hook up the throttle with spring to help it go back to idle when throttle is released, make sure you have some brakes and your set to buzz around.

This engine is a 4 stroke.

You can veiw my build on the moterized bike forum. I have not completed this build. 

 http://www.motoredbikes.com/showthread.php?t=20187