The engines I tested all would deliverway in excess of 9300 rpm on a 18x6 prop and the standard MVVS tuned header/pipe combination. That is very close to the max power that MVVS claims for the gas engine, and I am sure that with some adjusting of pipe and header length for best power at 9000 - 9500 rpm, their claim of 3.8 hp will be fulfilled.
MVVS is one of the few companies that do have a dyno test stand, and a dedicated person to operate the test equipment. They also are known to de-tune their engines somewhat (grudgingly) for a broader power range.
It seems that your engine is flawed one way or another, and this is an issue between you and horizon, not a general engine issue, and certainly not an issue on this thread which is about oil stains on the plane.