I'm going to go along with what others have said in that it would need to be amateur. I'm not really sure how these things work where you can have professional soccer/tennis/basketball players but not boxers.
Either way I think the rules would end up in some horrible watered down variant. And there would be a huge vaccuum before and after the event for a couple of months that would hurt the UFC.
Would it not be more realistic to have this as an amateur competition instead of trying to get the Lesnar's and the Machida's. That way it gets more respect as being a multi-layered credible sport instead of just the UFC. It also exposes the sport to a huge worldwide audience. The top guys can then go pro off the back of Olympic success.
Pick the Card Comp YTD: 61-30