In a medium saucepan, combine the orange juice, sugar, vinegar, soy sauce, ground ginger, minced garlic, and red chili flakes. Heat over medium and cook for 3 minutes, stirring occasionally.
In a small bowl, mix 2 tablespoons cornstarch with 2 tablespoons cold water until smooth to make a slurry.
Stir the cornstarch slurry into the saucepan and cook for about 5 minutes, until the sauce thickens. Remove from heat and stir in the orange zest; keep warm.
In a shallow dish, combine 2/3 cup all-purpose flour, 2/3 cup cornstarch, and a pinch of salt; mix well.
Whisk the eggs in a separate bowl. Dip each piece of chicken into the whisked eggs, then coat evenly with the flour–cornstarch mixture. Place coated pieces on a plate.
Pour 2–3 inches of oil into a skillet and heat over medium-high until it reaches about 350°F (175°C).
Fry several pieces of chicken at a time, without crowding, for 2–3 minutes per batch, turning so all sides become golden brown. Drain fried chicken on paper towels and repeat with remaining pieces.
Place the fried chicken in a bowl or skillet and toss with the warm orange sauce until evenly coated. Garnish with sliced green onions and additional orange zest if desired, and serve.