If a normal bath with hand soap, dish soap etc. doesn't take care of the rust, you can try something like Oxiclean. Toothpaste is also great for removing rust. Let the pony soak for a while and scrub. There's no need to worry if you can't get all of it off, though. As long as you let the pony dry properly before putting her head back on, she shouldn't develop any rust issues. Besides, if you use something non-metallic to keep her tail in once you rehair her, there isn't really anything that could get rusty.