I understand money is sort (coming from an environmental engineering student) but I know for a fact that Stihl has come out with new 4 valve weedwacker engines that are super fuel efficient. Being as it is a weedwacker I never have crunched the mileage numbers haha but w/ 1.9hp it'd make you rig boogie! love the idea.
Standard Engine Features of STIHL Landscaping Power Tools - STIHL USA