If you're China, then you should win, it's not too difficult.
Spend some time building up your cities initially, then build your military like crazy and go through the Korean cities regaining them, while disrupting the Japanese lines of supply with multiple caravels. Manchuria can be held by a few well placed units, especially Chu-ku-nu and cannon, as the Manchurians have no answer to them.
What's far more difficult in this scenario is winning as Manchuria on any reasonable level, and especially taking Beijing, which is very well defended by natural geography. Manchuria is out-teched on every level, and even spamming lots of units won't do against Beijing unless you get the tactics just right.