I strongly don't recommend YBR250 on this trip .
Trust the local wisom: Tibetan loves 五羊款(150-200 pushrod) which can easily blend into town w/o too much hassle to "police" as forigner in daytime. Every bike repair shop stocks parts . Easy to sell to "locals" if don't want to ride anymore.
The good reference material:
http://www.flickr.com/photos/amoleji...57608529571445