So, what can I shoehorn into a Fiat 124?

You can put anything you want in it. Fiats will accept anything, if you are willing to make a few mods. For example I have a friend with a '59 Fiat 600. It has a 2000? HP Dart small Chev, bored and stroked to 434CI, 2sp Powerglide, blown, runs on fuel, and has done 299.6 at B'ville. Probably 300+ this year. So be bold. You only get to go around once.