I don't know how the 81's mount, nor their original appearance. On the early cars, the radio position at the top of the center console had a metal plate with the usual rectangle and 2 circular holes pre-punched. I had to do a little opening up on the original openings, and cut the right sized holes in the original blanking plate (gave me a clamping effect).
As for knobs, etc, I searched this forum, ebay, all sorts of places for OEM Fiat radios, and got items close to an amalgamation of what I saw on the 'net. Might try that method; tedious, but worked for me.
Here's the best shot I have of my radio:
FWIW, I used the "Retrosound Faceplate and Knob Kit For 1957-77 Fiat, Alfa Romeo, and Volkswagen Beetle models (Black "Mini Euro" style plastic faceplate with black metal knobs) Item #: 068EUMBABK" from Crutchfield; their phone line tech found it for me.
Hope some of this helped....
Neil