Read this thread--the guy bought a used, Chinese-registered bike out of China, rode it in, had several problems, and bought a new bike, and rode it out to his home in Thailand.
He ended up with a Qingqi enduro. Qingqi has a mfr. deal with Suzuki, and manufacture Suzuki's small engines (i.e. DR200).
China or bust... - ADVrider
On this site MyChinaMoto.com - Chinese Motorcycle Community and Resources
there are several non-Chinese citizens as members who work and live in China who could help you out with a bike purchase.
If you are set on going through China, the most feasible way to do so is to purchase a Chinese-registered bike there, just from reading many, many posts from people trying desperately to find a magical "loophole" in which to bring your own bike into China. So Japan will probably not work for you.
From the guy's above thread, you can legally ride out of China with a bike (provided you haven't stolen it
)that you purchased there.
This should make it easier for you to plan a trip which includes, or begins in, China.