Is reciting Surat Ad-Duha to find lost things valid, especially given the absence of any action by the Prophet (peace be upon him) to that effect?

Allah has perfected the religion and His Messenger, peace and blessings be upon him, conveyed it in its entirety. It is not permissible to add to it. Innovation in religion implies accusing the Sharia of being incomplete or the Messenger, peace and blessings be upon him, of not conveying it fully. Designating the recitation of a specific surah, such as "Ad-Duha," when something is lost, or reciting Al-Fatiha or Ya-Sin for specific needs, is an innovation because it is a designation without legitimate Sharia evidence. Whatever was not part of the religion at the time of revelation is not part of the religion today.