I understand that you want to ship your bike registered in USA to Italy and ask for an Italian registration.
It's possible, but I don't know if it will be an easy job or worth it: consider you're coming from outside EU.
I need to do some research on it.
Anyway, the insurance for public-liability I pay yearly is around 500 euro for a covered risk of causing damages up to 5 Million Euro to people or 1 Million Euro to things.
This price DOES NOT include fir and theft: the price depends on how old is the bike but could be an additional 400/500 euro
Watch out: the price for public-liability is related to Rome, the city where I'm resident!
If you'll be resident in Leghorn, consider to divide it by 2 almost, much cheaper, small town, less accidents, less risks for the insurance company
For the aftermarket exhaust consider that the only one approved is the LeoVince, but it's a matter of luck if you meet the police while mounting something else. If it's not too loud you could make it!