Combine sugar and cornstarch in a medium saucepan, stir in grape juice, mixing well.
Cook over medium heat, stirring constantly, until clear and thickened.
Remove from heat, and stir in plums and almond extract.
Spoon plum mixture into a 10- x 6- x 2-inch baking dish.
Combine flour, brown sugar, and cinnamon in a small bowl.
Cut in margarine with a pastry blender until mixture resembles coarse meal; sprinkle over plum mixture.
Bake at 350Â° for 45 minutes or until browned and bubbly.