get a 4 stroke.
my cub 50 gets around 180mpg
also, gearing up, when possible is a great way to boost top speed and mileage. a double whammy. With a well tuned pipe and accordingly regearing I bet you could balance performance gains & losses pretty well (of both types)
You could also run lean which could boost mileage and drastically reduce lifetime. ha.
The usual stands too, good tires, tire pressure, good rolling bearings throughout, just totally minimizing any losses. Oh, and go on a diet.