My understanding is that when you leave you will need a passport, a yellow fever vaccination record, a permiso de conducir (available at AAA) and the title for your bike.
In addition to your passport and U.S. drivers liscense, You might also want (or need) other forms of I.D. like a birth certificate. You will generally need insurance for each seperate country that you enter (although there is some debate on this issue).
Apparently you also need a "Liberta de Pasos por Aduana" for south america, but this can be purchased en route, according to the AMH.