Dubro, Sullivan, byrons and possibly others in the past have made belt reduction units just for sixty size and larger glow engines.

I doubt that any are in current production because the proliferation of large gas engines pretty well killed the market. These units were popular during the early days of the popularzation of giant scale flying before the gas engines were available and worked well for the reasons you outlined and were reliable. I would love to get my hands on an old Dubro unit.

John