Traditionally, a KJF is preceded by Russia landing 2 Fighters in Kazakh and moving 1 tank to Yakut. This gives them the potential to hit Manchuria with 6 Infantry, Armor, 2 Fighters, which is pretty darn significant, IMHO.
Japan’s best bet, IMHO, if they suspect a KJF has started, is to work on getting units only from Japan and islands close to Japan (Philippines, Wake, Okinawa) that can be recovered without leaving the sea zone.
Your best strength here is to hope the allies screw up. Maybe England did a SZ 30 unification and went for E. Indies afterwards. Now you can lay into him with everything you have (which should be at least 6 fighters, bomber, 2 battleships, 2 carriers and, if you bought a submarine round 1, that too. Note, if they hit E. Indies not Borneo, then your aircraft will have to do the work, so plan on getting some fighters this round!) Still, the British fleet can be chopped up with fighters/bombers only (removing them from any action in Asia though).
But what do you do if the allies unify in SZ 45? Well, at least on the plus side, it’s SZ 45, not up near Alaska where they can cause SERIOUS damage to you by landing units in Russia!
In this case you have two options:
1) Try to out build the allies. (IMHO, this is NOT gunna happen!)
2) Try to take out Russia before the allies get there while building a little navy each round with Japan to prevent the allies from getting too close to you.)