The Soviets move first in AA42, they control China. China is now 6 territories and they produce infantry at rate of one per two (rounded up).
They have the existing rules from AA50 as far as deployment and movement restrictions.
Soviet units cannot enter any part of greater china ( includes Manchuria) until either Berlin falls or Japan attacks Soviet territories.
Chinese can enter Manchuria and reclaim it as control for production purposes
Starting army is one infantry each on 6 territories and fighter in Yunnan. I would give them an artillery. Fighter is replaceable if USA flys a new plane. ( use the rules under AAE for how exchange of pieces is made.)
forgot to add: Japan must garrison each Chinese territory with at least one land unit or China reclaims control.