You will need a much better ID system, and libertarians (sort of like you a bit no?), don't like ID systems. I favor a strong national very hard to fake ID system myself. But then I am not a libertarian.
So far as ID goes, all they need to do is modernize the primitive Social Security cards and require that everyone who wants to continue working past a certain date reapply for a new version. There are several different ways it could be phased in as well, so that you didn't suddenly have 160 million people rushing to get new SS cards all at once.