Some were modified for racing of course, both air and hydroplanes ran them as I'm sure you know. There was a Merlin powered drag racer, too, and I think there was a twin Merlin salt flats racer. But its also true that there are military modifieds, too. Ours was a "Ferry" engine, and our spare was a European Theater standard Merlin. The Mustangs here at home had yet another engine setup. The Ferry engine was tuned for longer air time, like a better mileage tuned engine in a car. It was set up for the long run across the Indian Ocean from India to Australia. (still drank fuel like crazy)
We came across some documents that described the "disposal" of P-51s after Korea. They were loaded on a barge and towed out in the ocean to be bulldozed off the barge. Its hard to imagine, but some of the finest aircraft ever built, Mustangs, Thunderbolts (Jugs), Skyraiders, Bearcats and more were simply shoved into the ocean, unstripped. Just dumped.