The sugar is crystallising. Try replacing the corn syrup with glucose syrup, or adding a little cream of tartar, both of which prevent crystallisation.
If you cannot find these ingredients, it will be hard to get this type of icing made.
The first thing is the liquid glucose. If you used any old thing labelled "syrup", chances are that it contained water, and this is what made your mix a sugar. Liquid glucose is mostly sugar, with almost no wetness to it. Substituting something very wet will not work.
Things which will work instead of glucose syrup are agave syrup, treacles, corn syrup, mollasses, golden syrup and honey. You should be aware that most of those will give you a more or less strong coloring and will have a bit of taste on its own. It depends on the type of your cupcakes whether the taste fits them. Corn syrup and golden syrup will be the best alternatives, but they are also less popular around the world.
The other thing is the sugar. It absolutely must be in a powdered form. Crystals won't work. If you have a food processor, just put the crystalized sugar in there and process it. Use it immediately, or it will fuse together due to air moisture.
But you may still be able to find icing sugar. It is just another word for powdered sugar, and it is used in Middle Eastern cuisine (your profile says Lebanon), for example for dusting lokum, but also in some halva recipes. If you can find a confectioner who does these things, you can ask them for a source of powdered sugar.
If you cannot find ingredients which produce an acceptable quality of icing, you can also try other types of icing recipes for decoration. Most do use powdered sugar for easier incorporation, but some are made with sugar dissolved in a liquid.
Best Answer
There is no reason a recipe like this can not be proportionally increased or decreased. Just figure out how much you need and adjust from there. So, in your example, increase everything by 1/3. ...but I don't see how doubling poses a problem.