I tagged a few threads discussing headers for you. Unfortunately, a lot of the development on naturally aspirated engines slowed down once the Abarth and later, the 500T came out.
http://www.fiat500usaforum.com/tags.php?tag=headers
There was a company with a promising header design that approached me about becoming a vendor on the forum about 2 years ago but they seemed to suddenly disappear. It was a surprise because the header was well designed and they were very excited about coming on board. I checked their site out and they never came out with the header. Too bad. Likely had something to do with the non-turbo 500 being discontinued in 2018.
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