NEW YORK - Mego International has signed an exclusive contract with world heavyweight champion, Muhammad Ali, - or the manufacture of a Muhammad Ali doll and boxing ring. At a recent breakfast meeting, here, attended by Ali and more than 700 toy industry buyers, Martin Abrams, president, the firm, said that the Ali doll will be ready for worldwide marketing this summer. The Muhammad Ali deal follows the recent announcement by Mego of both Sonny and Cher dolls and continues the company's program of creating dolls from popular TV characters, which have a large pre-sold market of children and adults.