What with Christopher Reeve up in the air regarding Superman II, III, IV, etc., the folks at Warner Bros, might consider signing up Marco Canestrelli, the would-be Man of Steel at left. When not defying gravity in Baltimore, Marco is one-fifth of the Flying Canestrellis, a trampoline act currently touring with the Ringling Bros. and Barnum & Bailey circus. Every night the 20-year-old trampolinist dons a Superman costume to vault four elephants. "I dreamed up the act long before the Superman craze," he says. Indeed, Marco has been trampolining for 13 years and is the only man ever to do a "sixtuplet," a 30-foot leap with six full twists in midair. This month he's going for seven. Who does this guy think he is anyway? Clark Kent?