While stock Fiat radiators are stout and well built, sometimes replacement makes sense. When I got my '87 Corsa X, the radiator was partially clogged and on long freeway runs would eventually run hot. However, the thermostat was sticking and also contributed to the problem, and was noticeable because it took a long time to warm up. So a new radiator was part of the fix, but the real fix also required a new thermostat.
Make sure your thermostat opens quickly at the correct temperature and fully, using both the gauge and an IR thermometer on the hoses. If it works right but still has problems, the radiator should be replaced. The VAS unit should work quite well, another option is the one at MWB which is the design that Bob Grasch had made and many of us have installed.
Whichever radiator you select, remember all the parts of the cooling system need to be good, including the cap on the pressure tank. My local O'Reilly Auto Parts loans out a nice cooling system pressure test kit that can help you chase down leaks and also test your cap.