Wonderful engine! I don't know how many times I've watched your video on youtube

I have an OS 5 cylinder radial (not started yet, need to build the glow driver first) and, as far as I know, all the OS multicylinder engines should be fairly close in peak power. So, looking at the propeller specs for my radial, I'd try a slightly smaller prop on your inline than a 22x10, like a 22x8 or 20x10. These seems to be, from what I've found, the largest props that these engines will take.

In any case i'm very interested to know the results of your propeller tests, so, please, go ahead