You can buy 704cc air cooled barrel and piston kits from Italy. I've also seen people use bis pistons and bored out a/c barrels.
I'm guessing the barrels would be the same on the Giardiniera as a standard 500. I know the head, carb, etc is a different set up.
I've never owned a 500 estate, but if I did I would be toying with a same idea engine upgrade wise.
You are right about the "air intake vent". I can't see one, but I can only see one side of the car. Shame there is no photos of the engine.
Not like I could afford to buy it, just curious