What if Tyson trained UFC?
Give him 18-24 months to get to a good level of take-down defense, and given his strong legs, and overall genetic brute strength and speed, I think Tyson could do well.
Without adequate take-down defense, I'm just not seeing it though.
Hard to say though. If he just opened up with a flurry of punches against Lesnar, and Lesnar couldn't secure a take-down quickly, things could turn against him, and in Tyson's favour.
Both are incredible in their chosen combat sports - I just tend to be of the mind that if boxers don't have good take-down defense, that they won't do well in MMA.