I was just thinking about this the other day, and I did a bit of googling on the subject. There are a number of dodges (just do searches on bicycle engine or bicycle motor).

Honda makes a nice little unit that mounts on a luggage rack-type thing and runs the rear wheel with a roller. This is a pretty common set-up, apparently. The French used to sell a handlebar-mounted unit with a ceramic roller that pressed on the front wheel; there was even a lever that let you push harder should it rain and the wheel start to slip.

Another company builds a kit to convert a number of different 2 or 4-cycle weed-whacker engines to bike use through the use of a centrifigal clutch mechanism.

Yet another offers a chain-drive system that uses a proprietary cogset with a motorcycle-type cog in place of the lowest gear on the cogset. The motor can be mounted in several different locations.

All of these things offer extremely high gas mileage (4-500 mpg!) but with the penalty of extra weight and complexity. Note that the weight is usually rather high on the bike, and might present balance/handling problems.

Hehe- My LBS had a cute little Giant cruiser on the showroom floor with a 50cc two-stroke and a little teardrop gas tank. No front brake!

Yes, purists, I know..... But the tinkerer in me has the urge....