I can pare down the rules to as simple as I can make them, and for the sake of any players, all dice rolls should be done by either the DM (preferably) or a designated dice-roller. This way, one person will be tasked with all the calculations, all the rolling, and no one else would have to worry about it and can focus on RPing. Considering that I have a bag of dice, I'll no doubt be doing that.
DnD might prove to be too complex, but I'll go look through my various systems and see what I can dig up. Another option would be to do a masterless system, but the only one I have on me is Dungeon Plungin', and that's more of a dungeon-crawl than anything. I may be able to convert some of the rules, though.
Actually, I'll talk to Ringman. He actually put together an arena-fighting/beat-'em-up system, so I'll ask him if he wrote it down anywhere. That might work, and as I recall, it was a very simple system. Otherwise, I'll have Wushu as a backup.