If rpm is not stable, the prop you use is too big, or the header is too short.
he prop must allow the engine to work above the torque peak, or the engine will loose rpm when the pipe gets hot.

I used a 24x10 3W wood prop in my test, which was a lot of load for the standard engine. It produced 6.81 hp @ 6600 rpm. With the same pipe setup, the engine produced 7.1 hp @ 7100 rpm using a 24x8 3W prop.

With the pipe set up well, one could expect about 1hp per 1000 rpm from the standard engine.