Plans to stage the hit Australian musical The Boy from Oz on Broadway appear doomed.         The sources say the producers want less emphasis on the show's main character, Peter Allen, leaving Australian star Todd McKenney, who was recently given a green card to work in the United States, a relatively minor role.     They also do not want the director of the Australian production, Gale Edwards, to be involved.     The musical's original writer, Australian Nick Enright, has also quit.     The show made $60 million in Australia and enjoyed a seven-month season at Melbourne's Princess Theatre.  It was due to open on Broadway earlier this year.