Two young British movie stars, Ewan McGregor and Charley Boorman, are starting out in April on an epic 20,000-mile motorcycle journey that will take them from London to New York by way of Europe, Russia and China, and then across the U.S. The book they are planning to write about the trip already has made sales here and in London. They will begin sending in an account illustrated with their own pictures in May, and the book is scheduled to appear, along with a TV series about the trip, this October. The buyer here, sight unseen, was Atria's Tracy Behar, and she got North American rights from Scott Waxman, acting on behalf of Peters Fraser & Dunlop in London, which has already sold it to Time Warner there.