There were *basically* two different 16v heads that can be identified
First is the Dallara 16v head which was completely custom made and looks more like an alloy box dropped on top of the engine and does not feature any of the curvy features of the Fiat head. These are ultra rare and worth a small fortune. The odds of seeing one of these on a road car is very, very, very short.
Second is one of the last versions of the engine built by Fiat themselves. The engine itself is one that I've only glimpsed but I can confirm it is basically the 8v block with a 16v head and featured all of the regular mounting points and dimensions. The engine I saw had been fitted to an X1/9 in Michigan and had been sourced from Fiats all weather/endurance testing centre in Detriot (yes it is still there and operational). The important thing though is that it was a prototype. The engine may have been used in a production car but I don't know what it is.
The abarth engines were actually proper twin-cams not a mimatch of the sohc and dohc engine designs. If your friend had a twincam in his car the odds are that the whole engine rather than just the head was a transplant. The 16v head mentioned above was not a direct fit onto the regular sohc block even though the block design was based on the sohc design.
