I don't know what a sundrop cake is. Is that a particular flavor or style of decorating?
I know there is a citrus soda called sundrop.. we're going to need some elaboration if we are to help. 
Got this recipe from Cooks.com I have never heard of Sundrop Soda though. Wonder if it is like a 7-Up? Hmmmmmm. 
SUNDROP CAKE
5 eggs
3 c. sugar
3 c. plain flour
Dash of salt
2 sticks butter
1/2 c. Crisco
1 tsp. vanilla
1 tsp. lemon flavoring
6 oz. Sundrop soda
Cream butter, sugar and Crisco. Add eggs and beat. Gradually add flour, flavorings and Sundrop until well blended. Put in bundt pan. Bake 315 degrees for 1 hour. Cool in pan 10 minutes.
Found this description;
****What hits you first is a decidedly strong citrus Soda scent. Almost a grapefruit like scent. On pouring you're greeted with nice, big bubbled carbonation that ends up adding a nice kick to the citric acid bite. Also, don't get thrown by the bits and pieces of citrus that float in these bottles, it's a natural result of using the Orange Juice Concentrate to craft this drink. It has a bright yellow/greenish hue that is a trademark of citrus Sodas, (anyone know why?) and they use Yellow 5 to get that colour, I can only wonder what the natural colour *****
This sounds more like a Fresca type than an orange type to me......Sheeeeesh! Now I am going to be on a mission to find a bottle of Sundrop! 
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