“Xanadu,” now playing at Surflight Theatre, is a patched-up jalopy of a musical, one that could crash through a guard rail and over a cliff at any second. Amazingly, it instead provides a hilarious 90-minute joy ride.

You don’t have to believe this reviewer’s opinion that the show, which opened on Broadway in 2007, could have been a disaster. The show’s book writer, Douglas Carter Beane, had Zeus (Jonathan Van Dyke) say as much in his script: “This is 1980,” the king of the gods tell his…