Mix juices, oil, and seasonings in small bowl. Reserve 1/4 cup of the marinade. Place chicken in large resealable plastic bag or glass dish. Add remaining marinade; turn to coat well. Refrigerate 30 minutes or longer for extra flavor.
Cook and stir chicken in large heated skillet on medium-high heat 3 minutes or until lightly browned. Remove from skillet. Add bell pepper, onion and reserved marinade; cook and stir 5 minutes or until tender. Return chicken to skillet. Cook 2 to 3 minutes or until heated through.
Spoon chicken mixture into warm tortillas. Serve with assorted toppings, if desired.