Have you seen these engines? http://www.scottellingson.com/brillelli_003.htm

They are about 2 1/2 pounds and from what I've heard they are a pretty sweet running engine.
I purchased one of the Toro off eBay for $40.00 delivered.
They are suppose to be easy to convert and Scott has them for $135 each for the low horsepower model
ready to go.

Just a thought