The process is different, from state to state. Because you did not mention your state of residence, sit is impossible for me to help you with the exact steps you need to follow.
In general, you need to go to your state's department of motor vehicles and get the form that handles the satisfaction of indebtedness secured by a subordinate lien or encumbrance on a motor vehicle.
Complete the appropriate form, and bring it to the deadbeat owner to sign. Bring the document to the DMV, and have the motorcycle placed in your name. At this point you are free to sell the vehicle or otherwise take possession of it.
You situation illustrates one of the dangers of co-signing on a loan. Anyone considering co-signing should be aware of a co-signer's rights and responsibilities.
I hope this information helps you Find. Learn & Save.