EDITED:

You get an international driving permit from your home country:

http://en.wikipedia.org/wiki/International_Driving_Permit

No problem at all to rent motorcycles and cars with that
*if you have the requirement in your original driving license*. 
While the convention says that you should have your original driving
license available I never have done that (could be lost) and I
have been never asked for that.

So strictly speaking legally *no, it is not possible to drive a
motorcycle with only an automobile license*.
If it is tolerated (some countries do not enforce street traffic
very strictly) I do not know, perhaps someone from Bolivia or traveling
to it could answer that.

As I have a motorcycle license myself I am sorry to say that, but driving
a motorcycle in a third-world country if you have no road experience is
asking to get killed. You need to drive carefully even here because you
have to get the "sense" how people react because you have no protection
(the leather gear and helmet protects against falling down (sometimes
deliberately to save your life)) against collisions and stupid drivers.