Drain as much oil as possible, then block off the intake and exhaust. I use metal block plates after removing the carburetor and exhaust pipes, or just remove the manifolds and block the ports well. Then turn the engine upside down. The residual oil left over after draining the crankcase will migrate toward the combustion chambers and keep the rings from rusting to the cylinder walls, and the blocked-off intake and exhaust ports will prevent humidity from cycling in and out of the cylinders with valves that are open. You can also loosen up the valve adjustments to make sure all the valves stay closed before inverting the engine.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!