Are you over 21? Have you had your driver’s license for over 1 year? In this case, yes you are able to rent a 50cc scooter in Greece. A class B license for over 1 years qualifies you for a 50 cc. However, for a 125cc you will need a A1 drivers license in Greece. I know in many countries you can driver either 50cc or 125cc with the class B license, here though it is not the case. If you are from some countries mostly South America you will need an internationals drivers license.
To obtain the international drivers license is not difficult and doesn’t take too long. You can do it online and will cost around 15 euros. iaalicense.com/en