Motorbike 'Letter of Permission' agreement

I'm not sure what you're talking about.  Are you referring to a situation where you rent a motorbike from someone in order to use it to move about?

In practicality, you don't need one.  These are the three things the highway patrol check for when they pull you over.

1) the A1 driver's license
2) the owner's registration of the motorbike
3) insurance for the motorbike

However, if you want a legal agreement, it's very time consuming.  They're just like any legal form.  Party A agrees to this, Party B agrees to that.  You sign two copies of it, give one to Party A and one to Party B. I'm sure you have to red stamp it at this agency then go to that agency to verify that the red stamp from this agency is true, then go to the other agency and translate it, then go back to another agency to verify the first agency is real.  You can spend several days "doing the legal right thing".  Red-Tape tourism can be fun.