I've found the puzzle source page, here's a Google translation :
"This car is a car that Salamander (meaning giant salamander ) , seems the car that person had made by hand from the '60s . This individual is so it was replaced by the body of the out beating aluminum in FRP. In space steel tube frame , the vehicle weight is a whopping 300kg stand . While on-board engine is air-cooled flat twin engine primary Publica , it is enough sport if this vehicle weight . Design is also great ."
So the car has a Toyota Publica engine. The website is indeed titled "Fiat 500 battle" (in Japanese), hence the confusion, but it deals with classic cars in general, not only Fiat 500s.