The O.S. long-stroke engines should be propped so that they turn in the mid-to-high 11,000's when leaned to just rich of peak RPM on the ground. Your engine sounds like it's running about the right RPM. They have been propped closer to 10,000 RPM for Pattern. A typical Pattern prop of that era was an APC 12 x 12 propeller. RPM would get into the mid-9,000's with a pipe.

To slow a model with the engine, you'll need a lower-pitched propeller. That means more diameter to keep the RPM in range. The long-stroke series of O.S. engines did not like higher RPM ranges, so keep it below 12,000 RPM, and you'll be OK.