What is the ruling of Islamic law concerning "Buddha" and "Zoroaster"? Was either of them a prophet, and was their doctrine distorted?

There is no evidence for the prophethood of Zoroaster or Buddha. Zoroaster was the leader of Mazdeism and falsely claimed prophethood. It was said that he was a sorcerer or stole knowledge from the Children of Israel. As for Buddha, his teachings were reformist and ethical, not based on a claim of prophethood. They are far from the doctrines of monotheism. He was a reformer who called for virtue and the abandonment of evil.