It will be fine on unleaded petrol.
I have no idea where that comment on exhaust manifolds or downpipes being affected by unleaded petrol comes from
We have had only unleaded petrol in NZ since the early 90s. In my personal experience, no FIAT requires leaded petrol.
The only practical problem with unleaded petrol is the variable amount of aromatic hydrocarbons (toluene, benzene) that are put in to bring up the octane level. These compounds make rubber and some plastics hard and brittle and may cause immersed items (e.g. o-rings, seals) to swell. Around 1995 in NZ, that caused many problems and fires - not just in cars or other vehicles, but in lawnmowers, generators, etc.
Practically-speaking, you just need to inspect all fuel lines for cracking and replace as you see fit. By now, few Uno Turbos would have the original fuel pipes - I'm hoping - mine were in a horrible condition seven years ago.
My parents had a 1970 FIAT 128 that ran on unleaded petrol for at least 15 years with no apparent problem at all, except that the low octane (91) sometimes caused pre-ignition problems due to the (crude by modern standards) contact-breaker-points ignition timing. I also saw one engine where pre-ignition had damaged the top of one piston more than the others (the distributor cam was found to be faulty). Even that piston damage had no real effect, though.
I know it says in the Haynes manual that only engines from 1988 run on unleaded petrol. I believe this is based mainly on changes to ignition timing to reduce the risk of pre-ignition. Although there appear to be changes to valve materials over the years (the valves look different), I believe that all valves in all FIAT engines (back to at least 1970) can cope with unleaded petrol. I could be wrong but ask yourself, what's the worst that can happen? The cylinder head may require rebuilding after more than 20 years of service? Who's to say that wouldn't have happened anyway?
I also feel that the spark-plug contaminating effect of lead or lead-replacement additives outweighs the possible benefit...
There are a number of people out there who consider that lead in petrol was merely a money-spinner for the companies that formulated the additive and its continued use was completely unnecessary. I'm probably on their side. I prefer the idea of a lower-octane petrol that burns cleaner without the toluene/benzene etc., and the Japanese seem to be good at making engines run well on 91. In the meantime, I run my Uno Turbo on 91 most of the time, as it doesn't seem to mind it.
-Alex
