Try to collect some worn our cassettes from bike shops or whatever, break the rivets and use the spacers between the cogs (it takes about 3 to get the correct amount) - at least then you're reusing stuff that would otherwise end up in the landfill, and it works as well as using the spacers you get in a kit.

That said, you probably have a freehub if it's the original wheel - in which case you're going to be better off getting a bmx freewheel to replace it, redish it and be done with it..