Little known fact.
Before FIAT signed a deal with Abarth they considered fitting a Moto Guzzi engine to make a hot 500. Moto Guzzi developed a special fan cooled V twin prototype (completely different to the motorcycle engine) and it was installed in a car. My father in law drove it and declared it dangerously overpowered, and believe me not much on wheels could worry him. FIAT dropped the idea because it would have required a complete upgrade of wheels, brakes, and suspension, and signed a deal with Abarth.
I have a suspicion that I know where the prototype engine is but I'm not sure how to find out for sure because the person I think had it died a couple of years back so I missed the opportunity to ask him.