Well, here's the thing about that. When people think about a reduction drive, they right away think about a long shaft and two bearings widely separated on the shaft, like the Toni Clark system shown below. That's going to be heavy no matter how you do it.

I once had an Ultralight Airplane that had a reduction drive that had just a short shaft to hold the prop pulley, and the pulley itself had two ball bearings in it. (The pulley shaft was anchored and did not turn.) It was quite light, and if that system could support 50 horsepower and a 5 + FOOT long prop and fly my heavy butt around for many, many hours, I think a similar system could be made to work on our little engines. That's how I would go about it.

AV8TOR