I'd take Kane for 50M in current form, in a positive attacking team he would excel, currently the team doesn't play to his strengths.
I also think a deal could be done for Kane simply because I don't see Kane being a 'Mourinho' player, you expect a player like Costa, Lukaku, Drogba with more strength is what he goes for, I'd even argue Ighalo is more of a Mourinho player than Kane.
It's not that Harry isn't good anymore, it's that Spurs aren't really playing well.
Kane is 26, still world class with a world class cast around him, only player I'd take in-front of Kane is Håland (age and his current ability).
Mason of course given the fact he's 18 by the time Kane would be in decline would mean Mason would move central and Kane would be moved on to a different club.
I certainly think the temporary dip in form isn't a bad thing, Kane is still world-class just you have a team that's playing with no confidence.