Any shaped pasta, such as penne, farfalle, and fusilli, can substitute for the rigatoni.
In a large pot of boiling salted water, add the rigatoni and cook until al dente.
Meanwhile, in a nonstick skillet over medium heat, heat the oil. Add the onion and saute until translucent, about 3 minutes. Add the sugar and stir until dissolved. Stir in the pumpkin. Add the chicken stock and reduce the heat to low. Cook until the sauce is thickened and the pumpkin is fork-tender.
Add the cinnamon, lemon juice, thyme, and salt and pepper to taste. Toss in the pasta and serve.