"The Merchant" will open at the Eisenhower Theater as scheduled, with Joseph Leon in the role the Zero Mostel was to have played. Previews begin Sept. 28 for a six-week run.

Mostel died last Thursday night in Philadelphia, where his illness had deferred the official opening. He played one preview performance there.

Arnold Wesker's drama is drawn from the major sources scholars asscribe to Shakespeare's "The Merchant of Venice," used many times in 15th- and 16th-century writings, "The Pound of Flesh," "The Three Caskets" and "The Elopment of Jessica." Shakespeare wove these unrelated plots into his romantic comedy.

The remaining Philadelphia weeks have been canceled, but enough was determined during several previews for director John Dexter to cut the play's running time from 3 1/2 hours to three. More cuts are expected.

As a result of these cuts, Sam Levene has withdrawn from his co-starring role of Tubal Di Ponti, now reduced to a single scene.

The new billing for the play will "star" the four leading players under the title: John Clements, the English knight making his American bow, Marian Seldes, Roberta Maxwell and Leon.

Originally cast in a supporting part as well as being Mostel's standby, Leon made his Broadway bow 25 years ago in "Bell, Book and Candle." He has acted there in "Seven Year Itch," "Pipe Dream," "The Gazebo," "Fair Game," "The Beauty Part," "Jimmy Shine," "Bad Habits," "All over Town" and "California Suite." He has appeared in regional theaters, television series and his role with 63 animals selling cat food is his most often seen TV commercial.