Then it occured to Mark/Mary.
The amulet exists, and it has magic powers.
Fairies exist and have magical powers (at least Mary did, though she didn't know if there were any naturally occurring fairies).
So magic is real. Well, if magic is real, there are probably other magical things in the world somewhere, right? Maybe one of those other magic things in the world *could* change her back.
"Amulet," she said. "I wish I knew where there's a magic item or place that can transform me, so I can become human again."
An image appeared in her mind. There was a pool off in the forest. It was about a mile away from the railroad tracks where Evan had found the amulet. It was surprisingly close--or maybe not surprising, since a pool like that would probably attract people who have things like magic amulets.
"Eva, you're going to help me..."
"Why me, Mary?" said the girl.
"Because you transformed me, and then I transformed you, and neither of us are going to change back unless we find this transformation pool. It's around a mile from where you found the amulet. I suggest you put me on your shoulder and get walking."