I've used a Rust-Oleum rattle can red oxide primer for rusty metal that seems an acceptable color. One thing for certain; the stuff is as tuff as nails. The only downside is it does take a day or better to cure to the point where the part can be handles if that necessary.
Try some DP74 it will give you the somewhat shinny look of the original paint (rather than the flat open finish of other products. You can tone or tint is a bit if you want to get closer to the original color (I do)