Preheat the oven to 325 degrees F. Lightly coat a nonstick 10-inch springform pan with aerosol baker's spray.
Melt the margarine and set aside to cool. Beat the egg substitute until creamy and light. Continue to beat and add the sugar and sugar substitute slowly, until the mixture is pale and fluffy. Slowly add the water, lemon juice, cooled margarine, vanilla extract, and lemon zest. Combine the flour and baking powder. Sift over the batter and fold in gently, using as few strokes as possible.
Beat the egg whites with the salt until they hold stiff peaks. Gently fold the whites into the batter, trying not to deflate them. Place in the prepared pan. Bake for 40 to 45 minutes, until browned and the cake tests done.
Cool in the pan on a rack. Remove sides of the pan and cut into slices to serve.