Maybe from the USA?
So ... it's not easy:
Vehicles, motor vehicles and trailers registered in a foreign country and which have already fulfilled customs formalities or those referred to in article 53, paragraph 2 of the Legislative Decree 30 August 1993, n.331, if prescribed, are allowed to circulate in Italy for the maximum duration of one year, based on the registration certificate of the State of origin.
As you can see you can circulate for a period of one year with the American bike with original plate but you must have these original documents with you:
a) TITLE (vehicle registration document)
B) Custom Bubble (shows that the vehicle is your property and you have fulfilled the customs duties and therefore VAT and DUTY)
c) Purchase Invoice
d) REGISTRATION CARD (it is our respective Italian stamp or property tax)
e) Insurance VALID ON THE ITALIAN TERRITORY
Obviously if you stay more than a year you must provide for the registration of the car with Italian license plate otherwise there is the immediate seizure of the vehicle with the relative administrative sanctions.
as for you, you must have the INTERNATIONAL DRIVING LICENSE (VALID).