Heat margarine in a work. Add beef and onion. Stir-fry over high heat, until beef is no longer pink. Stir in chili powder, corn, tomatoes, tomato sauce, sugar, oregano, salt and pepper. Bring to boil. Lower heat, cover and simmer for about 10 minutes. Top taco shell with ground beef mixture, lettuce and tomatoes. Top with coarsely grated cheese.