As already stated, flying at higher altitudes REDUCES output power from a normally aspirated plane engine.

You can do nothing to increase the power output, but you can help the engine not overheat, by reducing the load on the engine accordingly.

That is you can install a smaller diameter prop to reduce the engine load....

Now that said, bear in mind that the air is thinner anyway, so there is no real benefit in reducing the prop size, as the engine will have less air to bite into so to speak.

If you are attempting to get more power at higher altitudes, you'll either have to

- Increase nitro content ( this only works to a point... )
- Use a supercharger
- Get a bigger engine.

That's about it though.