Based on the fact that there are now weight classes in the UFC, Lee would have to fight at welterweight, currently the lowest weight class. So, he would have to face someone like Matt Hughes. Hmmm let me see Bruce Lee versus Matt Hughes. Lee was about 5'6" and 135lbs. He was lightening fast and could hit like a mule. If he bulked up I would suppose he would lose what made Bruce Lee, Bruce Lee. So we have Bruce Lee in his prime versus the Matt Hughes who fought Royce Gracie. Result: Hughes by submission or referee stops contest. Bruce is one of my heros and one of the reasons I studied Wing Chun for 10 years but you have to cut through all the entertainment show biz fighting and realize that a good big man always beats a good small man. Size does matter. Especially in a contest with rules and rounds.
Now Bruce with some mma training going up against a man of his own stature, man I'd hate to be his opponent.
And finally, what if Bruce Lee met up with Matt Hughes in the park and for whatever reason got into it. Then I would say that Matt Hughes would not know what hit him. Using vital striking points, shoes, no gloves and no rules would be advantage Lee. Especially because Matt Hughes would be facing the kind of attack he never has seen or has had experience combating. Kinda reverse of what Lee would be facing against Hughes in the octagon.