Preheat oven to 400°F; halve squash lengthwise, scoop seeds, drizzle with oil, and season with salt and pepper.
Roast cut-side down on baking sheet 40-45 minutes until tender.
Fork-scrape into strands; press with paper towels to remove excess moisture.
Mix strands with garlic, Italian seasoning, and 1 cup marinara; divide into shells or transfer to baking dish.
Top with chicken, remaining marinara, mozzarella, and Parmesan; bake at 375°F for 20-25 minutes until cheese melts and bubbles.
Garnish with basil; serve.