What can i use as a corn syrup substitute in icing?
Making your own sugar syrup should work.

Corn syrup can be replaced by a sugar syrup. Combine one cup of pure cane sugar with 1/4 cup of water and heat over a low flame. Cool and use directly in a recipe. If preferred, cover the pan for three minutes to help remove sugar crystals, then add 1/4 teaspoon of lemon juice or cream of tartar to the pan and stir frequently until it reaches the soft ball stage (a drop of the syrup will form a ball when immersed in cold water). Cool and store in a sealed container at room temperature. It should keep for up to two months.
Alternatively, add granulated or brown sugar to a recipe that calls for corn syrup, cup for cup, then increase the amount of liquids used in the recipe by 1/4 of the amount of sugar added.

For your use, you'll probably want the liquid version.