Above and below are pics of the engine driven portable air compressor conversion produced by Gordon Smith & Co. of Bowling Green, Kentucky through the 1950's.
This conversion could be applied to either a Model A or Model B Ford engine, and primarily consisted of a special cylinder head arrangement.
The eBay pics below are of a Gordon Smith & Co. air compressor cylinder head. Thanks to Randy Fink for allowing the use of his photos on Ford Garage.
This compressor engine runs on cylinders 1 and 4, and pumps air on cylinders 2 and 3.
The Gordon Smith head uses the Model B style three-bolt water pump provisions.
Note the revised spark plug locations relative to the valves and combustion chamber.
The rubber hose is a balance and return tube for the water cooling between cylinders 1 and 4.
The air compression cylinders use reed valves, not poppet valves.
Note the revised combustion chamber shape of cylinders 1 and 4. They are quite unlike either Model A or Model B combustion chamber shape, and would presumably require a special head gasket.