Why not cut the head in half and block off the water passages then mount a thick milled out aluminum plate over the other 2.
and mount commercial air compressor poppet valves like those used on a Quincy air compressor
route the 2 not used injector lines to the fuel return
now you would have a self running air compressor that would pump air on every up stroke of the 2 pistons
I rebuilt a Miller Air-pac 400 amp welding machine that had a 3 cyl air cooled Deutch engine #3 cylinder was done this way it produced 40 CFM @ 120 PSI while welding
you could add an un-loader valve that would idle the engine down at a set PSI or after a period of time that the pressure stayed at full pressure
Engines have been converted this way since the days of the model T ford