Somebody was making a conversion kit to allow a weed-whacker engine to power a bike. There was a centrifugal clutch assembly that replaced the bolt-on "arm" of the weed whacker, and a mount arrangement that you attached to a regular bike luggage rack.
A motorcyle-type sprocket replaced the lowest gear on the cassette.

Looked like a cute idea, but I don't know about that screaming 2-stroke behind you....