How did the Hashirama and Madara statues at the Valley of the End come to be?
From what I can tell, these statues are not protected or enchanted. After the Naruto vs. Sasuke battle at the Valley of the End, the status were damaged, and later repaired. I believe it was a reconstruction job, much like Leaf's faces on the mountain, or the Kage statues at the Sand.

Words of Madara
I honestly don't think Hashirama would give a damn enough to make statues, let alone self-repairing statues. He wasn't the sentimental type.