Baror,

More than a couple people on here have bought a bike in China an exited to another country. Foreigners regularly ride between China and MOngolia so ti must be possible. As a matter of fact, I think there is a post on here of someone trying to organize that exact same trip but not riding around "in" China. We'd need more details about your plans.

Leaving one country is entering another and you'll have to pass through both. Perhaps the most simple is to buy a second hand motorcycle which already has plates and registration and ride it around and out. There is lots of discussion in MyChinaMoto (check the Regionals-Asia section) about this so read around if you haven't already.

CC