Your best bet is to try and find a Fiat dealer, who still has the old microfiche and slides. If they can look it up for you, you might get lucky and find a dealer with stock. Or even Fiat themselves, may still keep stock in Italy.
But you need to find a UK dealer who's been around for at least 30 odd years, an assume they haven't binned the old microfiche and slides!!
As unfortunately any Fiat's pre-1990, Strada, Regata, Mk1 Uno etc.. are not on the
ePer system.
I doubt there is a part number on the cable, but might be worth a look?
At least that way if you got the original part number, you'll be sure it's right. Even if it's changed part number, which can happen sometimes. The dealer should still be able to check with the number you got, as it should tell them what the superseded part number is.
Failing that, there are companies who will repair or reproduce cables if you have an original as a pattern
Sorry it's not exactly a straight up result on a cable, but hopefully you'll get one soon enough.
I have had similar hassle with cables for my Uno 45, so feel your pain!!