Japanese are more protective of their privacy than most Americans. Unless they have a strong social media presence, you might as well hire a private detective.
I don't think it's common at all for Westerners to change their names when going to or marrying in Japan. Mukoyoshi is the word for a man who is adopted into his wife's family, when there are no male heirs, only daughters, for a valuable business or property. The Muko usually change his name to his wife's family name.