For the meatballs: Preheat the oven to 350 degrees F. Coat a baking sheet with nonstick cooking spray.
Mix together the chicken, pumpkin, Parmesan, onion, parsley, salt, pepper and garlic in a large bowl. Cover and refrigerate 20 minutes.
Shape the mixture into balls using a tablespoon and place on the prepared baking sheet. Bake until cooked through, 12 to 15 minutes. Increase the oven temperature to 400 degrees F and spray a 9-by-13-inch baking dish with cooking spray.
For the casserole: Meanwhile, bring a large pot of water to a boil. Season generously with salt. Add the pasta and cook 3 minutes less than the package instructions. Reserve 1/3 cup pasta water and drain.
Toss the pasta with the marinara sauce and pasta water in a large bowl. Transfer to the greased baking dish. Nestle the meatballs into the pasta. Top with the mozzarella and sprinkle with the Parmesan.
Bake until the cheese is melted and bubbly, about 20 minutes. Let stand for 5 minutes. Serve with grated Parmesan.