The cast for the American premiere of Tom Stoppard's "Hapgood" has been set. Roger Rees, who will re-create the role he originated in the London production, stars with Simon Jones, Judy Davis, Richard Lawson and James Lancaster. The Ahmanson production will open April 12 at the James A. Doolittle Theatre in Hollywood. Low-priced previews begin April 1. Information: (213) 410-1062.