Don't have the game yet, but the general rule of thumb should be: look for the propeller pitch command and together with throttle adjust it according to the speed and
RPM gauge. Try to keep the RPM in the area (range) where the engine can give you most power.
With the mixture you don't have to hit the mark (probably), but if you over rev = end of story.
How much you can push the engine, and squeeze out of it (Ilya himself said that overheating is terrible in those early planes) you will have to find for your self.