Any engine will run on half its cylinders without a lot of modification- - - -just disconnect the spark plug wires to every other cylinder in the firing order. No camshaft modifications are required. On the compressor conversions, it was necessary to keep the gas/air mixture out of the cylinders used to compress the air. The valves are blocked shut on the cylinders that do the compressing, and the lifters are removed. The spark plug hole is enlarged for a bigger thread, and a fitting is installed with a one-way poppet valve with a very weak spring and an exhaust port, so air can be drawn into the cylinder, compressed, and sent through another one-way valve to a storage tank. Every downstroke of the piston draws air into the cylinder, and every upstroke compresses. Some of the more sophisticated rigs had a custom-cast cylinder head with bigger ports for the cylinders that were doing the compressing. A smaller carburetor was fitted to handle the reduced airflow to the cylinders that are still running. I think the modern unit that uses the 302 Ford as a base engine uses custom cylinder heads. I don't remember which company sells that one.
With a little tinkering, I believe a modern 4-cylinder engine from a compact car could be converted into a pretty effective compressor, using the same methods that the Model A conversion used.
Jerry
