About those crickets. Sorry, would have helped if I could have.
That's a good lookin' 500. Despite their popularity, large production numbers and good parts support, I'm not aware of any super 500 sites based in the US. I think there is a good forum from the UK. Mirafiori is real popular and interesting, but seems to favor 124 128 131 and even 850.
The Yahoo microcar board is active, with quite a few Fiat 500 afficianados <microcar-minicar@yahoogroups.com>
Join the Vintage MicroCar Club, if you haven't already. The club publication is really beautiful and informative, may provide a few more resources.
You'll get it back together and running sweet, I have no doubt. I don't have one at the moment, but they are fun, easy to own micros. You'll enjoy it even more after having the opportunity to repair that engine.
Cheers, Alan