I keep thinking that the US and the UK, especially the US, were supporting the Chinese before the Pacific War started. Claire Chennault was basically running the Chinese Air Force from 1937 or so on, and the US was supplying a fair number of aircraft, both as ready to fly aircraft, and as assemble in China planes. I would have to look up the exact number, plus the AVG was being recruited and trained before December 1941. Not quite sure how to reflect that in the game however.
Chinese troops did move into Burma, both in 1942 and later in the war. For them to go anywhere else, i.e. into India or the Soviet Union, would require them to cross some very bad territory. Restricting their movement to areas outside of China to Southeast Asia might be a good way to treat that. That is basically where they can move in the Pacific game, as north and west are not an option.
One thought that I did have for the Chinese fighter, to distinguish it from the US planes, is to use the P-38 from the Pacific set, and use the Hellcat for the US fighters, as that seems to be the aircraft used in A&A50. Then you could have a Chinese-specific aircraft to use and continue to replace.