Place squash into a large pot and cover with water; bring to a boil. Reduce heat to medium-low and simmer until tender, about 10 minutes. Drain and mash squash in a bowl.
Stir 1 cup corn flakes, margarine, eggs, sugar, seasoned salt, and black pepper into mashed squash. Transfer mixture to a 2-quart casserole dish. Sprinkle about 1/4 cup cornflake crumbs over squash mixture.
Bake in the preheated oven until bubbling and topping is lightly browned, 20 to 25 minutes.