THE US CONSTITUTION'S BICENTENNIAL promises to be a mega-event on the order of this year's Statue of Liberty celebration. The observance kicked off Sept. 17 with ``Miracle of Philadelphia,'' an exhibition at the Second Bank of the United States. It will accelerate with that city's July 3-5 Freedom Festival parade and concert, and it will peak next Sept. 17 with a televised reenactment of the ratification, in which President Reagan and former Presidents Carter, Ford, and Nixon are expected to participate. GOYA CAPTURES WASHINGTON'S ATTENTION this month, with exhibitions at the National Gallery and Corcoran Gallery and with a world premi`ere Nov. 15 of Gian Carlo Menotti's opera ``Goya,'' starring Pl'acido Domingo and Victoria Vergara. LAUREN BACALL will return to Broadway next spring as Princess Kosmonopolis in a new production of Tennessee Williams's ``Sweet Bird of Youth.'' The two-time Tony winner was last seen on Broadway in 1983 in ``Woman of the Year.'' RADIO CITY'S chief will leave Jan. 1 to head Madison Square Garden. Richard H. Evans, who has managed the mammoth movie palace since 1980 and is credited with saving it from bankruptcy, will be replaced by James A. McManus, currently the hall's vice-president. PETER BROOK'S `MAHABHARATA' will begin a world tour next summer with stops in London, Zurich, Los Angeles, and New York. The nine-hour theatrical adaptation of the Hindu epic will be performed in English. The original French version premi`ered at the Avignon Festival in 1985.