EDITED:
You can get an international driving permitinternational driving permit from your home country:
http://en.wikipedia.org/wiki/International_Driving_Permit
No. It is no problem at all to rent motorcycles and cars with that if you have the requirement in your original driving license. While While the convention says that you should have your original driving license license available I never have done that (could be lost) and I have have been never asked for that.
So strictly speaking legally no, it is not possible to drive a motorcycle motorcycle with only an automobile license. If If it is tolerated (some countries do not enforce street traffic very very strictly) I do not know, perhaps someone from Bolivia or traveling to to it could answer that.
As I have a motorcycle license myself I am sorry to say thatthis, but driving a a motorcycle in a third-world country if you have no road experience is asking asking to get killed. You need to drive carefully even here because you have have to get the "sense" how people react because you have no protection against collisions and stupid drivers (the leather gear and helmet only protects against falling down (sometimes deliberately deliberately to save your life)) against collisions and stupid drivers.