I think even current retired Pov would be a problem for Zhang. Prime Povetkin beats all of the current crop except for Usyk and Fury. Both men would be 50-50 against him.
Uh, er, I seem to remember that Wlad beat him for the belt, and then fought Povetkin. He was at the end. That is my recollection.
Povetkin UD's this, maybe even KO's. Much better boxer. He caused Joshua big problems at 38, in his prime he'd KO Joshua, who would KO Zhang.
Zhang is a dangerous opponent for anyone. Povetkin is definitely the more skillful and accomplished fighter. If the fight were to happen now, I would favour Zhang but Povetkin in his prime would be a different story.