That's a heck of a car Dave. I've always loved those, and that one looks like it's not far from being a great driver.
I've been talking to a guy about an NSU Prinz IV. If you're into little cars, you need to get one of those. It looks a lot like a miniature Corvair.
You might try a wanted ad in the classifieds for the parts you need. If you know any cross-overs for the parts, that would help too. Do you know what other years and models might have the same air-cleaner?
I was wasting some time waiting for the Formula 1 race to start, and noticed that Ricambi Fiat 500 lists the air filter lid portion. I've never done any business with them, nor have any affiliation. I just noticed it and thought I'd pass it along.
http://ricambifiat500.com/category/spare-parts/engine-parts/