Re defrost vents
I think they are same as Ferrari Daytona and Alfa Montreal. But they are also same as (or are so close as makes no difference to a Fiat 124 Spider). I bought a set off eBay for £30 (c $36). Guy has lots Fiat parts. Same diameter and with same tabs as above, not sure if the Mangusta were unique but somehow I doubt DT wouldn't have just raided the parts bin.
There is also a guy in Holland who I buy Alfa Montreal parts from, and could have the part if anyone needs some.
cheers