Sam,

Just a couple things you may already know or other people are probably going to point out to you.

#1 You will need a residence permit to obtain a license, registration and motorcycle in China. If you're not working and living here, you can get someone else to buy the bike for you, but that will make it quite difficult to get it out of the country and into other places. The license issue has been beat to death on MCM, I'm sure you can figure out what is best by searching the site.

#2 Tibet is off limits to foreigners. You will find it very difficult to just slip through without some serious local knowledge and perfect timing. If you're OK with hiring an official (expensive and restrictive) guide, Tibet shouldn't be a problem.

Good luck with the quest.

Cheers!
ChinaV