Preheat the oven to 375°F (190°C).
Cook the jumbo pasta shells in salted boiling water until al dente, then drain and rinse with cold water.
In a bowl, mix pumpkin puree, ricotta, ½ cup mozzarella, Parmesan, garlic powder, sage, nutmeg, salt, pepper, and egg until creamy.
Spread 1 cup marinara sauce evenly in a 9×13-inch baking dish.
Stuff each pasta shell with the pumpkin ricotta mixture and place in the baking dish, open side up.
Pour remaining marinara sauce over the shells and sprinkle with remaining ½ cup shredded mozzarella.
Drizzle olive oil over the top.
Cover with foil and bake for 25 minutes.
Remove foil and bake for an additional 10 minutes until cheese is bubbly and golden.
Let rest for 5 minutes before serving.
Garnish with fresh parsley or sage leaves if desired and serve warm.