Preheat oven to 425°F. Spread squash on a large, rimmed baking sheet. In a small bowl, stir together chili powder, garlic, oil, 1/4 tsp. salt and 1/8 tsp. pepper; pour over squash and toss to coat. Roast squash, turning once, until blistered in spots and tender when pierced with a knife, about 20 minutes.
Warm a large skillet over medium-high heat. Working in batches, toast tortillas, turning once, until charred, about 2 minutes per side. Transfer tortillas to a clean kitchen towel and wrap up to keep warm. Repeat with remaining tortillas.
Fill tortillas with squash, avocado, cilantro and onion, dividing evenly. Serve with lime wedges.