Hinges on several factors, such as whether or not he can strike, and with the most heavy emphasis on his capability to recover from blows.
If he has a strong chin, I doubt that anyone in MMA today or in the past could hang with someone of his physical prowess and mental capacity for sport.
Rulon Gardner (Greco-Roman Gold, beat Karelin) fought against Yoshida (Judo Gold) in Pride Shockwave 2004. Rent it at Blockbuster or Netflix.
This fight tells me that takedowns on Karelin would be impossible for any of the top grapplers in MMA, and that he could power out of submission attempts. Striking ability and desire to finish the fight are questionable, but Gardner exhibits outstanding stamina.
Unfortunately, a Karelin fight is akin to an ALi, Jones Jr, Sugar Ray, etc fight. What if any of the worlds greatest athletes trained? Until pay and popularity is ridiculous, the very best will always go to other sports.