Heating the plastic makes it soft and pliable, meaning you can squish the face back into its correct position and then stuff the head with something to keep the shape. As the plastic cools and hardens it should go back into its original shape.
This technique also works with other squished bits of pony anatomy.
I use a hairdryer on a low setting to heat the plastic. You have to be really, really careful here as the plastic will heat quite quickly and there is the potential to burn yourself. Pony hair can also be melted with excessive heat so you need to be quick.
You can also use boiling water to soften the plastic but I find that this cools too quickly. It is safer though.