When comic strips from America were outlawed in Italy by Mussolini in 1938, Federico Fellini struck a blow for fantasy by writing his own version of Flash Gordon. Now the movie maestro is back in comic books: His Trip to Tulum is a surreal graphic novel based on The Journey of G. Mastorna, a movie the dream-driven director abandoned mid-production in 1966 after he had a nightmare about the set's destruction. Twenty years later Fellini turned the story, inspired by the hallucinatory voyages of author Carlos Castaneda, into a newspaper serial for Milan's Corriere Della Sera, and illustrator Milo Manara suggested pulling together the pieces into a graphic novel. ''He needed a little coaxing,'' says Manara. ''But as he saw the characters come to life, he got more excited, his instructions got more specific, and by the end he wanted to do everything himself.'' The 71-year-old director of 8 1/2, whose The Voice of the Moon is currently in production, wound up overseeing even the comic equivalents of camera angles, and Fellini-philes will have no trouble recognizing his touch: The director ''plays'' himself and the hero is the spitting image of Marcello Mastroianni.