Problem solved ! .............. don't drive in the rain in any pre 1980 "collectible" .......... just promotes rust !
If you must drive in the rain, and no parts can be found or made up on a lathe, consider inserting a thin "roll pin" right thru the worn out knurled piece after using commercial/heavy industrial grade Superglue or epoxy resin.
Leaving both ends of the roll-pin a little exposed by a few "thou", can help further with "anchoring" the wiper arm on the knurl.
Perish the thought, but if you're desperate enough and the parts are very worn, tapping a thread at top of pivot to hold arm on with, say a 6mm bolt with "loctite", may be further help after the above may be considered insufficient.
I go to incredible lengths NOT to get any of my cars wet; EVER. Water gets into crevices you have zero chance of drying, and there's your rust breeding ground. My first car that I still own since 1967, has no more rust than the little it had in 1967, as it never gets wet. It's always garaged and always covered and might get ** "dry cleaned" 4 times a year
** (squeezed out) sponge whole body with a reputable car wash, and then clean off with Chamois (no water or detergent leaves the sponge)
I wouldnt keep doing this for decades if it didnt work