Preheat the oven to 350 degrees F (175 degrees C).

Prepare chicken: Place beaten eggs in a shallow bowl. Combine cornstarch and garlic salt in a second shallow bowl.

Dip chicken in egg, then press in cornstarch mixture to coat.

Heat oil in a large skillet over medium-high heat. Cook chicken in hot oil until brown on all sides, sprinkling with onion salt, garlic salt, and pepper while frying, 3 to 5 minutes. Place browned chicken in a 9x13-inch casserole dish.

Make sauce: Combine sugar, vinegar, ketchup, and soy sauce in a bowl. Pour sauce over chicken.

Bake in the preheated oven until chicken is no longer pink in the center and the juices run clear, 40 to 60 minutes. An instant-read thermometer inserted into the center near the bone should read at least 165 degrees F (74 degrees C).