An IDP stamped "B" for passenger cars doesn't allow you to ride a 50cc scooter even though a Japanese license for a car will allow you to. You must have the IDP stamped "A" to go 2-wheel.If you're working in Japan, you might want to get a Japanese license which will bring you into their insurance coverage. An IDP doesn't give you any insurance.
You must always carry your home country's license with an IDP and show both if a "shirobaiku" asks you for your "menkyo."
The IDP is only good for a year maximum for anyone living in Japan and must be converted to a Japanese license if staying longer than a year.
30km on a 50cc and your hands will be numb from the high frequency vibration. Mileage varies by scooter or cycle because of tire sizes and weight differences. I don't remember mileage as I last borrowed a 50cc in '05. They go a long way on a tankful, though. It's also cold in the winter, unless you're in Okinawa.