Preheat oven to 350 F.
Cut squash in half lengthwise, scoop out all the seeds and fibers. Place on a baking sheet, cut side up and roast about an hour or until the skin gives easily and inside is tender.
Remove the spaghetti squash and fluff the spaghetti-like strands with a fork. Leave the squash in the shell. Spoon about 1/2 cup marinara sauce, 1/2 cup mozzarella, and chicken pieces into squash and season with garlic powder, basil, and Italian seasonings. Place in oven another 10 minutes until cheese is melted.