This is something I wrote some time ago. It's not exactly a hack, I think, because all the changes are superficial; it's more like an extensive reskin of the weapons system.

The inspiration came years ago when I read the first (second?) edition of GURPS Martial Arts. There's a session toward the end where they give suggestions for campaign settings that justify dedicated martial arts rules. One of them was a post-apocalyptic setting. It's ideal: guns and weapons aren't readily available, and society is reduced to its primitive forms where violence is currency.

Here is the PDF.
http://www.mediafire.com/download/xxb3a3wtoxd65et/Fists_of_Asphalt_v1.pdf