I don't think any Labour leader could have won 1997, but I do think any realistic Labour leader would have - Beckett, Prescott, Cook, Brown, Smith etc. None of these would have positioned the party substantially to the left of Blair, given a '94 or '92 leadership start date. Smith certainly would have won, though with a probably seriously reduced majority over Blair - likely high double figures rather than high triple.Conversely I'm much less sure post-2019 of the old left consensus that any Labour leader could have won against Major in 1997, or even that it was nailed on for Smith. There's probably a horribly depressing timeline to be written called 'One More Heave'.
Obviously though if the chronosphere deposits Benn or Corbyn into the leadership, it's game on and the fear of such would give the Tories the shot in the arm they needed.