How hard would it be to just find a compressor pump? You could look for something made for 3-phase that the average guy wouldn't be able to run and then swap the engine in. I would think in the long run it would be less frustrating.
As far as converting an engine, try looking for something with OHV. Pull the rocker arms (or camshaft if OHC) to keep all the valves closed. Pull all the plugs, and pipe the plug holes to a new pair of pipe manifolds, using check valves at each cylinder. One from filtered air supply to a tee to plug hole, and from tee to tank line, check valves directly attached to tee. The issue with this is that you have # of cylinders x2 of check valves to possibly go wrong, and the plug holes are not a standard plumbing size. You would either have to tap the plug holes for pipe or make up special plug thread adapters.
Your other option is to make a new head, reinventing the wheel with reed valves etc.
Or, you know, just find a 20hp compressor head that would actually be made from the ground up for what you want and have spare parts available.