I have an Irvine 15 , that came from JUST Engines It was shipped with an extended prop nut and the head was already "chopped' , milled off fins for aircraft use, rear exhaust, carb is the same/ It runs great but not in the 20K range. HAVE YET TO PUT IT IN A PLANE martin
It was offed in both configurations do not rembember the prop it got 16K, most likley a smaller one would increase revs iIt was converted to diesel
with a custom head also