Preheat oven to 325 degrees F.
Butter a 12 cup bundt pan with 1 teaspoon margarine and set aside.
In a small bowl mix together the 6 teaspoons of Ener-G egg replacer with the 8 tablespoons warm water, stir to combine ingredients and set aside.
Into a large bowl mix together the flour, baking soda, baking powder, salt, cinnamon, ginger, allspice, nutmeg. Set aside.
In a large bowl using an electric mixer; cream the margarine, brown sugar and sugar. Add the sweet potatoes and beat until light and fluffy. Stir together the Ener-G and warm water then add to ingredients in mixing bowl, mixing well.
Add flour and 1/2 cup orange juice to batter: 1 cup flour then 1/4 cup orange juice, beginning and ending with the flour. Stir in vanilla (and nuts if using). Pour batter into prepared bundt pan and bake approximately 1 hr and 10 minutes or until a tester inserted into the center of the cake comes out clean. Remove from the oven and let cool for 10 minutes in the pan.
While the cake is resting in the pan; in a medium bowl combine the confectioners' sugar, 2 tablespoons orange juice and orange zest. Stir well to combine and make a tight glaze, adding more orange juice 1/2 teaspoon at a time to achieve a pourable consistency.
Turn out cake onto a plate. While cake is still warm, drizzle orange-sugar glaze to evenly coat the cake. Cool completely before serving.