Look into the California Commuter. It was built in the 1980s as a high mileage project and set some Guinness Records for fuel economy. It's built around a modified Honda S90 engine and is pretty basic, but a good starting point for ideas and development. I bought the designer's instruction book with plans, intending to build it with a few modifications. My commute has changed but I am still considering building an all composite electric version. His website is
Doug Malewicki's patented inventions and engineering involvements..