To be honest mate it sounds like a head gasket issue if it’s loosing coolant that fast. One thing I would do is to get a new gasket kit and get the head skimmed. Also it might be worth stripping and rebuilding the engine whilst reconditioning it, give it a good clean, if it’s an old car it deserves some TLC.
the HG was done when i got it then pulled the engine out
https://www.fiatforum.com/cinquecen...977-cinquecento-899cc-head-gasket-change.html
its a timing chain but i have no idea for the aux belt
Ash