So I'm not the only one...
I think you should focus on 125p and 126p (of course) and also on FSO Polonez, that was based on Fiat 1500 platform (same as the 125) and at the beginning had Fiat 115 engine (later Citroen).
Look for the nice Estetyka period models. Something has been done more recently by Welly, but in slightly larger scale than 1:43.
I don't think Tarpan and Nysa are Fiat based, however.
And if you go back to pre-war time, there are also the interesting army vehicles made by PZInz: Fiat 508 Lazik and Fiat 518 Mazur. But you can find models only in 1:35, 1:72 or 1:200 for role-games.
Also the (almost) well-known WZ.34 armoured vehicle used Fiat Balilla engine, at least in the 2 latest series (not the first one with tracks, that was based on Citroen mechanics).