No, they don't. They really don't.
The beauty of the home recordings -- of which "500 Miles" is one of many -- is, indeed, their homespun simplicity.
There is something subtle and haunting about the arrangements as they were recorded all those years ago. I especially like the sound of the piano on many of the recordings; dark, sombre, unknowable, it's reminiscent of the sound Lennon achieved on his seminal track "Imagine".
Any overdubs or re-recordings would destroy the intimacy of what Elvis and the guys (and gals) achieved.