Try this article's advice, and be careful not to strip it. IIRC, it's an aluminum plug. If it's not, heat might help, but you'd have to be careful not to heat the plug itself- just the aluminum around it. I'm not sure if I'd want to go that far, so that's a last resort IMO. Try the mix mentioned above, and let it soak in. You might want to try some minor acid action as mentioned. When you re-install the plug, I'd use loctite- blue or green- to prevent a future galvanic reaction.
Best lube for Rusty Nuts