I ran into a similar issue 10 years ago. A magazine had published a short article I had submitted, without prior notification it was accepted, and without prior payment.
In my case, I was ignorant of standard industry practice, and had no leg to stand on, but I did do some research.
Since your original publication was 5 years ago, you presumably did not register the article with the U.S. Copyright Office within the required period, so the damages you can be awarded will be limited. But if you can produce clear evidence of the infringement (and it sounds like you already have it), you should have no trouble collecting damages.
Contact a copyright attorney. An initial phone consultation will probably be free. If the potential award isn't worth the attorney's time, go for the maximum small claim. It will cost the magazine 10 times more than they would likely have needed to pay you if they bought the article legit, just to defend the claim, regardless of whether they win or lose. Heck, they'll spend that on transportation alone, if the court proceedings are on your home turf.
Check out magazines that could conceivably have purchased the article. Find out what they pay for similar articles. Find out whether they would pay the same for an article that has already appeared in another magazine.
That's your minimum damages. Since they modified the article without your permission, and in so doing introduced errors which could conceivably have damaged your reputation, you may be able to claim additional damages. If you can, get a copy of the print edition that your plagiarized article appeared in. (Any good reference librarian can help you there.) No judge would rule against you.
Best of luck. You've got thousands of writers pulling for you.