Don't worry, ponies are pretty sturdy and putting them back together isn't nearly as difficult as taking their heads off! (That sounds awful, lol
)
I second SweetNightmare's advice!
If the head is still firmly glued on, I use an x-acto knife to carefully slice around the plug. Or actually I don't slice so much as gently press the blade down, trying to work in sections and bending the head back to see the spots that are still glued. Sometimes the glue cracks simply when you twist and bend the head. G1s are so much easier than other gens so you shouldn't have much trouble here! You can use a duller knife (like a butter knife or something) and/or a flathead screwdriver, too.
The easiest way to put the tail back in is to skip the washer and just push the metallic part keeping the tail together back in (sorry I can't remember if there's a term for this :'D ). Or you can use the thread method.
You'll be fine, I promise, and Peachy will too! Just let her dry a couple of days after her spa before putting everything back together. Good luck!