In article .com,
says...
I know that horizontal shaft engines are easiest to convert and
I should be able to find an old roto tiller or something similar,
but have no idea how to make a working drive train from motor
to wheels. I was thinking of making the frame out of wood and
re-inforcing with angle iron since its much easier to shape.

Tiller engines in the 3 to 5 HP range are plentiful and easy to
work with. A simple, common drive consists of a centrifugal
clutch with integral drive sprocket on the engine and a larger
sprocket attached to one of the rear wheels. Don't skimp on
brakes, very important. Northern Equipment carries a variety
of stuff from engines to wheels and plans to kits. For safety
and durability, I'd avoid wood for any of the frame or suspension.
You can do a lot with angle iron using a good drill and hacksaw.