Piston, cylinder, and "crankcase" material selection would be critical. Cox did great with steel on steel for their engines, but used hydraulic valve precision on the cylinders. If I worked at Cox, I would use cylinders, pistons, and rods like those used in their engines. I would use a double ended rod to a wobble plate with two ball ends like the rod used in their engines. . However, each piston would only complete 1 stroke per revolution.
If I made the engine I would make the cylinder from chrome plated brass with high silicon aluminum pistons and "crankcase". The pistons would have a lapped seal like current high performance model two strokes. The valve and swashplate could be case hardened steel. A wobble plate design might be better, though. Cooling will be an issue especially in larger sizes. I would expect friction and leaks would make performance lower than conventional four strokes and much lower than model two strokes. Read the axial swashplate pump topic to get an idea of the problems involved.
Lohring Miller