What Midnight_Reaper said is probably the best solution. Just keep on going the way it is, re-dividing the sides from the victor.
Another option would be to essentially do a re-setup of the game post 1945 depending on how the game played out. This would be a rather herculean task I would think, as it would require multiple setup scenarios. What I mean is:
Different setups for different outcomes. If Allies/Comintern win, redistribute the map/money/pieces in a way that seems accurate to you (probably mostly on historical 1945/Cold War lines). If the Axis wins, same thing, split it up.
This will also depend on the level of victory I would think too. As in, if the Axis wins and totally runs over the USSR, but doesn’t take over the Western Powers, you could have a setup based on that. Reconfigure standing armies and placements, etc. So while Japan may split from Germany/Italy in the Axis, the US and UK would also still be alive and kicking. Maybe you’d also allow the USSR partisan rolls. Would be a miracle for them, but there’s nothing that says they don’t get lucky and could re-spawn everywhere and wreak havoc on the occupying forces.
Same with a Cold War scenario. Maybe Axis powers could be given partisan type abilities, undermining the Western Allies or Comintern occupations.
As I said though, this would require making multiple alternative setup scenarios that maybe you wouldn’t want to do. If maintaining the forces you had at the end of the game is desired, this wouldn’t work. Or, maybe you can just redistribute those surviving units, and give the other surviving powers money/units to balance the game out.
I do wonder if this is in any way what HBG has in mind with their spinoff games. 1861, 1914, 1920’s, 1936, Cold War, Modern, Future, etc. Will they all be linked based off of previous games, or are each stand alone games. I’ve always been a bit unclear which it is.