I think you will need to turn in the neighborhood of 8000 to make power.

Here is data on another brand of 25cc engine (McCulloch) that might be helpful.

Thought everyone might like to know that Ken’s muffler is good for 1100 RPM on a stock 25cc engine with no other changes.

Started with a stock engine.
16 x 8 MA K series prop – 6700 RPM

Muffler only change but did turn the carb screws
16 x 8 MA K series prop – 7800 RPM

The other engine with modified exhaust duration turns 8200 RPM.

Bill