Even if it was by Michelangelo, is it really a Michelangelo if 60% of it is restoration?

According to an article in the July 1, 2013 issue of The New York Times (“Trumpeting a Michelangelo (Cue the Trills of Dissent)” by Elisabetta Povoledo), at a meeting in Florence organized by the Opificio delle Pietre Dure on June 24-25, 2013, the restored sculpture of the young St. John the Baptist that was housed in the Holy Chapel of the Savior in Ubeda, Spain where it was badly damaged during the Spanish Civil War, was presented as a work of Michelangelo. Assuming that this claim of authorship– which was first proposed and shot down in 1930—is correct, the sculpture as it exists today is 60% restoration. How much or little original material must a work of art contain for it to be considered from the hand of the named artist?