I did one of these a few years' back.

The performance of mine was almost identical to that of my Zenoah 23 ... same prop and same rpm.

I made a record of the figures.

Smart 18 x 6 wooden prop, standard exhaust, large-bore carb ... 6,700 rpm.

I squeezed out 6,900 rpm after modifying the exhaust; but there was a noise penalty, of course and a deterioration to the engine's idling. That's around 1.3 hp, I estimate.

The engine flew my 15lb Stampe reasonably well.

If you have room for a tuned pipe, expect to increase the power output by around 40%.