I think the problem with a 1939 varient is it would have to many special rules and seem almost too automatic for the 1st few turns to be any fun (for me at least).
That being said if you wanted to do it and avoid as many special rules as possible:
1)maybe have a 4th ally listed as “other allies”: It could represent France, Poland, Greece, The Dutch, Belgium, and Norway. They would probably only get 1 IC in france and you would expect them to fall fairly quick. If you wanted, you could probably just merge it all with the UK and still only have 1 IC on London.
2)Also, maybe other than a small force in the Phil and maybe a token pacific fleet, have the US wiped clean of any forceson the board
3) Make sure Germany and Russia cant reasonably attack eachother anywhere for 1 or 2 turns (except maybe fin/kar).
4) Make sure the Japanese have enough attacks to be obvious and attack to where they would roughly be by 41 (hong kong, dutch forces, chinese forces for example)
Off the top of my head that is the best I can think of without it becoming a “rules game” like the US can only attack odd number of territories if the Japanese player says “I’m a little teapot” 5 times fast and for three turns the US can lend X amount of dollars to nation X. All this and Germany gets to attack in a special (but loveable) way for 2 turns.