In a large skillet, cook chicken and onions until chicken is no longer pink. Add water, tomato paste, oregano, pizza seasoning,garlic powder, and black pepper. Mix well and bring to a boil. Reduce heat and simmer ten minutes; set aside. Preheat oven to 375. Prepare a 16" pizza pan with cooking spray. In a mixing bowl, combine cooked macaroni, two ounces cheese, and eggs. Mix well. Spread macaroni mixture into pan. Bake for 15 minutes. Spread pizza sauce over macaroni. Sprinkle top with mushrooms, pepperoni, and remaining cheese. Bake pizza for 20 minutes, or until heated through.