The hardest part is the ricotta - as far as I know, there's no soy substitute for that. If there IS, then that makes everything super-simple.
2 cups cooked thin spaghetti
1 ounce grated Parmesan cheese use soy parmesan
1 egg, beaten use 2 egg whites or 1/4 cup EggBeaters
1 tablespoon plus 1 teaspoon softened margarine This doesn't really serve a purpose, and you could probably omit it if the fat concerns you. Otherwise, use a light dairy-free margarine, like Smart Balance Light.
2/3 cup part skim Ricotta cheese use light tofu, mixed with maybe 2 or 3 tbsp of soy parmesan to make up the taste. I would also consider combining it lightly with the meat portion, rather than layering it, if you find tofu too nasty-tasting.
2 teaspoons margarine
1/2 cup each diced onion and red or green bell pepper
1 garlic clove, minced
6 ounces cooked ground beef, crumbled use ground turkey
1 cup canned whole tomatoes, drained and chopped (reserve liquid)
2 teaspoons tomato paste
2 ounces mozzarella cheese, shredded use soy mozzarella