Get the AM to get their finger out and in 1940 you can realistically have cannon armed Spitfires fully sorted and being spat out of Castle Bromwich at the same rate as achieved in late 1941. Now that is hard to beat IMVHO.
Yep have Lord Beaverbrook pick up the phone and have 'that' conversation with Nuffield 6 months earlier and get Castle Bromwich stood up at full production at the beginning of 1940.
Should have enough Spits to fully equip FC by May 1940
This allows Hurricanes to be sent overseas in Greater Numbers and allow more Airframes for the real enemy - the Royal Navy
Anyway as to the OPs question
France - Have the Dewoitine D.520 simplified and production stood up earlier - the French Airforce could have had 300 odd in service for the Battle of France OTL but had sent most of them back to be 'perfected' to the latest standard. Should have accepted them and replaced with later production. Sell surplus Moraine 406's to 3rd parties such as Norway, Netherlands and Greece.
Japan - 1940? Get the Zero and Zeke ASAP they were probably the correct A/C given the large areas that had to be covered. Perhaps produce a version with maybe a bit of armour/Self sealing tanks? Would a radio kill them?
UK - Get Castle Bromwich stood up earlier - should have enough Spits to cover all needs. Switch Hurricane production over to Whirlwind and Spitfire ASAP. Sell surplus Hurricanes to 3rd parties such as Norway, Netherlands and Greece....and maybe a few to the Fleet Air Arm. Stick HS 404 in everything.
Germany - Get the BF 109 to carry more fuel (the Germans will be attacking after all) - I personally hate the canopy - hat of to the pilots for fighting in the damn thing - so improve the visibility if possible. Add the Gondola cannon ASAP otherwise it will be out gunned. With a longer ranged 109 - limit production of the 110.
Italy - Get that wing issue sorted earlier on the C.200 wing resolving the Auto Rotation issue - don't halt production in early 1940 and spam out more of them with radios and ignore the pilots and install a full cockpit. Add more guns.
US - Get production of the P40 stood up faster by all means tinker with the Supercharger for better performance at altitude - but spam it out - get as many P40s out in the field to as many Allied nations as possible as quickly as possible. As for the naval fighter - Buffalo and Wildcat are probably still being developed in 1940 but perhaps ensuring a folding wing is developed in 1940 rather than having to wait until 1942! And 4 guns is fine.