Zeref doesn't start with time stop in character (iirc, he's never used it in battle), and especially not White Zeref.
White Zeref is kinda cocky, he had millions of haxy methods to end Natsu after gaining his new powers, but instead chose to ram his fist through his chest, which he survived, and then proceeded to get into a FIST FIGHT with Natsu. Which he lost (which is BS by the way).
So, when he does go in for the kill strike, Kenshiro will easily be able to counter with speed equalized, precognition, and talent for reading a fighters movements, it'll be omae wa mou for Zeref.
Of course, his time rewind based regen will let him survive it. Though it should be noted that it appears his natural regen has suffered, as he stated that after his beat down from Natsu it would take him a few minutes to heal, whereas before he used to heal much faster.
So now it all comes down to what happens next. Will Zeref lead with time stop? Death magic? It all really depends on his next move. (This is of course assuming Ken is just gonna stand still while Zeref regens)
If it's death magic, Kenshiro can dodge via intangibility and precognition (he receives a brief vision of the future of a persons attacks or read a persons aura to predict their movements) with Muso Tensei (which can activate automatically if the user is in danger, even if their unconscious, he also possess instinctive reactions), and then erase Zeref from existence, therefore defeating him.
If it's time stop, then it's a whole different ball game, one in which Zeref wins.
The thing is, Zeref rarely ever uses this ability, so I see Ken winning 7/10.
