You need a ATA carnet to cross the border and come back again. In theory you can get ATA in every ATA office across China but prolly will have to get it either in some tier 1 city or at the border. I tried to get one for Mongolia and it didn't work out in the end cos they tried to rip me off. If you get an agency involved you will be fine. I did not have time to do that after my pitiful experience. Agency will charge you between 300 and 1000 kuai.
ATA itself varies from country to country. Mongolia is 40% value of your bike when new. It's a deposit tho.
You also need an invoice to prove the value of your bike

If you don't have the ATA you might succeed in leaving China but your bike will never come back