I am not sure if any one has done one before but how about a snow blower engine. Just the other day I picked up a new/broken Yard Machine from the local small engine shop for $30.00. It needed a new piston and ring which was $30.00 so I have $60.00 into a new snow blower. It is a Tecumseh 98cc that would be easy to convert and not too heavy with a CH ignition. It will need a Walbro carburetor also. My .02
Tim