[Hal]

Without some pretty major modifications you are going to be hard pressed to make any significant improvements in fuel efficiency with a two-stroke engine. The problems is that the for part of the cycle the intake and exhaust ports are open at the same time and a portion of the incoming fuel/air mixture goes right out the exhaust port. Expansion chambers work to increase power by using a reflected pressure wave to reduce the amount of charge that escapes into the exhaust port, but they are tuned to specific RPM ranges which reduces their effectiveness in daily riding situations. Some manufacturers attempted to address this problem by using things like sleeves and other valving arrangements on the exhaust port. By the time you do that you have increased the weight and complexity and lost some of the advantages of two-stroke design. The other problem, of "dirty" exhaust is that of having the lubrication for the cylinder and crankshaft being burned along with the fuel.
A solution to both problems would keep the incoming charge out of the crankcase altogether and using direct injection of fuel, hence my comment about "major modifications". It would require some sort of blower for scavenging air and a high pressure injection system, as well as replacement or modification of the cylinder so that the incoming air goes directly into the cylinder, and a way of lubrication the rod and crankshaft bearings. There seems to be a few companies that are working on this or producing kits, and there are snowmobile and jetski engines that are direct-injection two strokes that could be modified/adapted to a motorcycle frame.