I like the China mod Idea, would like to give it a try.
However, I think these might be some possible options for the House rule.
1. China can only purchase Infantry and artillery, perhaps add a new unit, Cavalry! at 2,1,2,4?
2. U.S. is and Russia can fly aircraft to chinese capital, on subsequent turn they become chinese.
3. Japan should start with a bomber and an extra sub or destroyer in 41 and an extra carrier and f2 fighters in 42 to balance this out.