One thing I've done in the past was to tighten everything up, after adjusting the location of the arm carefully as it should be, was to then drill a small hole down between the side of the shaft and the arm mounting lug. Then I ran in a very small self tapping screw down that hole so that it acted just like a woodruff key to pin the arm onto the shaft. Or, I might have tapped that hole since I have had small taps, etc, but I don't remember now.
Worked fine for years after that, and you could still pull things apart if needed. Nothing irreversibly damaged.
Or, you could drill a horizontal hole through the arm mounting lug and the shaft, and install a bolt going through, as if it were a "cotter" pin. Once again, nothing irreversibly damaged.
You do have to make sure that everything is adjusted exactly correct.
BTW, did you have a chance to to talk to RR about the 164S pieces they might still have?
89 Milano (wife's daily driver since 1989, Shankle Sport)
91 164S (my daily driver since 1994)
94 164LS (~Q) (trip Alfa since 2000)
72 Morgan 27 (water time since 1976)
previously owned since 1964:
62 Morris MiniMinor 850, 67 Austin 1275 Cooper S (Downton 3/4 race), 64 Giulia Sprint GT (1st red one made), 72 Fiat 128 Sedan, 75 Alfetta Sedan, 78 Alfetta Sedan, 78 GTV, 81 GTV6, 86 GTV6