Ten years ago I did the math, & did this...

http://www.staton-inc.com/store/products/Honda_GX35_INSIDE_DRIVE_Gear_Box_3=
5_cc_1_60_Four_Cycle_kit-391-15.html

Mine was a little different; what was offered at the time.

Works great. Quite hilly around here, too. 160-200 mpg, depending
on gearing, who's riding it, & so on.

Comments: I started off with a 36-spoke rear wheel. After I switched
to 48 I never broke another spoke (Velocity Deep-V rim).

I'm a big guy. YMMV.

I've got an 1:2 drive out of the gearbox - It'll climb a 7% grade @ 10 mph,
without protest. Top speed 17 mph  -  IMO, enough, considering bicycle
brakes...

I ended up with 2 front wheel folding wire baskets, & 1 in the rear on
the left side. Very handy.
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