In a 5- to 6-qt slow cooker, combine the sugar, vinegar, soy sauce, ginger, garlic and red pepper (if you prefer more spice, use 1/2 tsp here). Add the chicken and turn to coat. Cook, covered, until the chicken is cooked through and very tender, 6 to 7 hours on low or 3 to 4 hours on high. Twenty-five minutes before serving, cook the rice according to package directions. Meanwhile, cut away the peel and white pith of the orange. Cut the orange into thin half-moons and place in a medium bowl. Fold in the scallions. Using two forks, gently shred the chicken into medium pieces and stir into the cooking liquid. Fill the lettuce leaves with the rice and top with the chicken and the orange mixture.