If you were to "hot rod" it, a fun choice would be teh later V6 from an Alfa GTV6.
But you can easily make good power from a later Alfa 4 cyl DOHC, too.
The successor to your car was and is a pretty popular race car:
To be honest, I'd take the Giulietta, drop the later 2.5 liter V6 with aftermarket cams, dual exhaust, flare the fenders over some fat Panasports, and make a dual purpose street/track day car. The Alfa guys would love it. There are enough stock examples around... Make it kind of like an Abarth race car had the V6 been avaialble to them back then.
The convertible versions are turned into race cars all the time: