Place sweet potatoes in a large kettle and add water to cover. Cook over medium heat until barely tender; drain. Peel and cut potatoes into fourths. Arrange in a medium size casserole. Preheat oven to 350F. While potatoes are cooking, combine sugars, ginger and cornstarch in a microwave safe measuring cup. Add juices. Blend well. Place in microwave on high and cook til sugars are dissolved and mixture starts to bubble; Stir in Amaretto and margarine. Pour sauce over potatoes. Bake uncovered 50 to 60 min or until sauce is thickened.