Too funny. I always thought everything was available in the USA!
Yes, icing sugar is powdered sugar, otherwise known as confectioner's sugar. Certainly you have that!
Custard powder is, well, powdered custard. Here's a picture of what I'm talking about. Also, I checked on-line and found that vanilla pudding mix can be used as a substitute. I've never tried it, so can't say what kind of substitute it would be.