Dax may be able to answer this question himself, but I am not sure about how accurate an estimate you can get.
A lot of what will determine the power of one of these engines are the details of the build, such as any porting or port matching that may have been done, as well as what sort of intake and exhaust are installed.
you have mentioned that you are building a compressor in another thread, assuming that you are still working on that project might I suggest directly contacting an engine builder (like Dax, who you've mentioned or any other person who builds and sells these motors) and asking them about getting a motor set up for this purpose. Ideally you would want a motor built to be producing a lot of power but running at a lower RPM compared to most motorized bicycles. chained or belted to your compressor with the right gearing.
you might encounter cooling issues with the motor stationary, and noise issues depending on your exhaust system.
as awesome as what I think you are trying to do here is, I would be doing you a disservice if I did not point out
this which is a tiny honda four-stroke used for things like what you are doing. I've seen a few using a quick google search going for $200-$250 new.
whatever you end up going with, good luck with your endeavors!