The rust is caused by a reation between the metal in the bearings, crankshaft, cam, lifters, etc..., and the nitric acid that is formed as a byproduct of combustion of nitro fuels and moisture.
The best way to combat this is to run the engine completly dry at the end of a flying session by keep trying to fire it up until it will not fire anymore to insure no unburnt fuel remains in the cylinder. Then inject your favorite after run oil into the tube connected to your crankcase vent and flip the prop several times to get the oil thoroughly distributed in the engine. Most my planes are hung from the tail to help let the oil run from the main bearing end to the front end where the cam box and lifters are and of course the front bearing. Every so often, I pull the valve covers to check the valve lash and give it a good healthy squirt of oil, like Bill Robinson mentions, an upright Saito can get a little dry up top sometimes.