Bay Street Theatre in Sag Harbor will launch its 2013 Mainstage season with “Lend Me a Tenor,” a comedy by Ken Ludwig originally produced by Andrew Lloyd Webber on London’s West End in 1986, then on Broadway in 1989. Directed by Don Stephenson, the cast will include Judy Blazer, Scott Cote, Betsy DiLellio, Donna English, Nancy Johnston, Noah Plomgren, Steve Rosen, and Roland Rusinek.