Present a bit of the old goofy code that is (mostly) self-contained -- maybe add comments about global variables to explain what they contain, if necessary. Then also present the equivalent chunk of logic as written in your current "non-icky" style. It might be instructive if the old version contains a flaw of some sort that is fixed in the new version.

The task is to determine whether the two pieces of code really do the same thing, and in the process of discussing the snippets, get a feel for which of the styles seems more familiar to the applicant -- and which one is easier for him/her to understand (might not be the same as the more familiar one).

As others have said, the key is still in the discussion more than in the "factual" answers. If the newer, better style confuses an applicant, but they really do get the older style, this might not be a bad thing, especially if they show signs of catching on to the newer one. You just have to figure out how much training you're willing to invest in "re-education", given that they have a leg up on the older coding style.

On the other hand, if the newer code is a walk in the park for them, but they can't work out how the old code relates to it, they might not be as valuable when it comes to doing maintenance and retooling on the old code base, and now you have to figure out how much time you want to invest in just ditching the old code and re-creating an app from first principles (not quite "from scratch", but almost).