I found it, at last!
It's a Parisotto Fiat 750 Sport from 1952 (chassis #5062652).
Engine is a Fiat 750cc with a Siata B type head (is the "s" on the side grill connected to "Siata"?). That particular car is one of a series of 9 cars. It raced the Mille Miglia in 1952 (number 2400), driven by Gino d'Angeli with co-driver A. Gasparini (did not finish).