I'd presume light heavyweight, if we're sending Mikkel back in a time machine. Could easily be super middleweight, though, with Jake brought forward in the continuum. LaMotta weighed in somewhere in the 160's more often in his career than anything else. He produced his best quality at MW, though. He struggled with and lost to lesser man than Kessler. I'd pick the Viking to outpoint the Bull.
There almost certainly wouldn't be a KO but if it's at 168 I'd have to favour the rangier, more experienced (at the weight) Kessler. Having said that, it's definitely not a sure thing as I always felt Kessler is remarkably open to the body, something the Raging Bull would definitely take advantage of.
Kessler has a small chance based on his slightly larger size. But, the gap in toughness and tenacity is a mile wide, so I would go with Lamotta by decision.