You are not far off -for that engine
They were designed to run at about 10000 -typical for Zenoah industrial engines
as a rough aprox, you will pick up about a grand in level flight BUT on climbing stuf you will be closer to the readings on the ground
Anything over most 16x6/8 props will likely reduce power -on that 20

I presume you are not running tuned exhaust - the in cowl mufflers reduce max power so don't expect a larger prop will increase performance - typically on the smaller gassers they have to rev pretty well.
On my 25/26 cc gassers -on tuned pipes - I propped on the ground for about 8500-9000 rpm-props 18x8 wood
These engines both had recording tacs built into the ignition and maxed out readings (no dives!) were10000 + which was about max power on gasoline do not over lean the Zenoah- it should hold rpm from level flight-thru a loop- this means a teensy bit rich for level flight.