Here's a clue as to how to protect from rust. Control line fliers never have a problem with rust. Why? Because almost invariably they (we ) use a reasonable amount of castor in the fuel and at the end of every flight the engine runs dry of all fuel at full throttle (barring crashes ). To do something similar for RC, at the end of the last flight for the day keep the engine running, go to full throttle and either pinch or remove the fuel line and wait for the engine to stop. If you pinch the line then keep it pinched for a few seconds after the engine stops to let the residual tank pressure bleed back into the muffler then completely drain the tank.

If you use an all synthetic fuel then you must use an after run oil because synthetics don't stick to metal, they slowly slide off leaving bare metal exposed to air and moisture. Castor (like all good after run oils) is attracted to metal like a magnet to iron so can't run off. Technically it's known as being polarised.