Three sets of plans for home garden tractors. All are of similar design: air cooled engines, smaller wheels, lightweight but capable machines.
The plans use junked parts to keep costs low. In the above picture, those are Buick rear wheels, Briggs and Stratton engine, Ford steering and transmission, and a pre-WWII Austin rear end.
Mostly one needed a hacksaw and a welder to make these simple projects and keep costs low. The transmissions helped to maximize the lower horsepower.