Here's a company that will turn your weedwhacker into a model airplane engine or a motorized bicycle. Do the world a favor and opt for the new 4-stroke engines from Honda and others when buying small engines (mowers, whackers, etc..). Less polution, less noise, and you don't have to mess with mixing oil and gas.
Better yet, try out some of the cool new electric automatic and push mowers. I was surprised to discover that battery powered mowers aren't a new concept. Here's a svelte looking battery powered mower from the 1950's.