Actually, rust is really easy to take care of! Be brave and take her head off! this can be a bit difficult if the glue isn't loose, but you just need to put her under a stream of really hot water and wiggle the head to loosen the seam. also pull, hard ^^ (this is usually the most painful part for me). Then take the tail off from the inside (I use tweezers for this). Once the tail is out, get rid of the rusty washer, just throw it away.
Now to clean the tail (and the inside of the pony if necessary). Basically, it's enough to just use common toothpaste! What I usually do is, I put some toothpaste, take an old toothbrush and try to get rid of as much rust as I can in that go. Usually, you are left with stained after then. In that case, I put another does of paste, rub it in and let it stay for a while (30min-1h) and then rinse and scrub till it comes off. Repeat as necessary ^^
Once you´re done cleaning the pony, putting the tail back can be tricky. I usually heat the pony hole with a hot stream and push till the tail goes in

I never replaced the washer though as the tail usually stays in for me, but some recommend using zip tie.
Hope this helps! good luck!