Do some reading on the board. You will find out that:
1. It is possible to rent a bike without leaving your passport, but you may have to leave a large cash deposit for the value of the bike.
2. You need all sorts of documentation and permission from the owner to take the bike out - registered or not.
3. If the bike is not registered, there is no guarantee that they will let you cross.
Details have all been posted before. Do some research.
And then, write up your experiences for the next guy.
BobS
"Life's journey is not to arrive at the grave safely in a well preserved body, but rather to skid in sideways, totally worn out, shouting "...holy shit...what a ride!"