The stock 600D tranaxle should be just fine with around 100HP according to everything I've ever read. I used to know someone with a Berlina Corsa that had a hot Fiat 124 twincam in it, with the stock transaxle. It handled the power fine. His car used to really "squat" when it took off from the start at autocrosses.
With 100hp + in a 600, you will want to beef up the suspension. You also better look at reinforcing the chassis too, to be able to handle high horsepower.
There used to be a kit available in the US (PBS?) for installing a 128 SOHC motor in a 600 or 850. I don't think weight is an issue, but you'd have to deal with the engine being longer.
I recall Al Cosentino having a photo in one of his books with an injected motor in a late Berlina Corsa (circa 1972). I can't find the photo at the moment. He claimed it was producing 125HP, but it was not legal in the US to race, so he raced it in Canada. That would be an awesome setup, but definitely not for the street!
You can get some pretty good horsepower from 1050cc (or 903cc block bored/stroked to 1050cc) engine with all of the right stuff in it. Take a look at the Scuderia Topolino website for lots of good info. I love his website for all of the great tech. info.!!
Gil