You are right, I just checked out the 127 Sport online. That is not a model I was familiar with--never saw it in any of the rear engine Fiat or Abarth books.
The engines may not have been that rare, but they haven't been made for so many years, and there were none ever imported by Fiat here to the US.
The 1050 engine was intended for the Fiat 127 (and Autobianchi 112 Abarth) and has an overhead cam and no pushrods.
Perhaps one could add, that Abarth made a small series of Fiat 127 based rally equipped cars for a rally series in France. The model is sometimes referred to ing. Colucci's last work for Abarth. I have one needing restoration.
The overhead cam Fiat 127 1050-engine is not for rear engined cars like 500, 600 or 850 - all having pushrod engines.
It is true that the 903ccm version for the Fiat 850 can be confused with the 903 ccm engine for the Fiat 127. In fact they share quite some parts - but the layout is different. In the rear engine cars the engine is installed longitudinally - but transversely in front engined Fiat 127s and Autobianchi 112s.
I have seen Fiat 850 blocks (903 ccm versions being 5 mm. higher) converted using parts from a Fiat 127, but it is extremely comprehensive. The 850 engine versions turn ccw, so a correct cam is essential - the Fiat 127/Autobianchi cam item can't be used.
The usual solution is to use the internals of an Abarth 58hp engine used in early Autobianchi 112 Abarths (longer stroke for 985 ccm and better torque) - and find a ccw cam (try Berni Motori in Italy for a 850/1000OT cam). Usually an electric fuel pump is needed.
The beautiful finned valve cover from the Autobianchi 112 Abarth has an engine oil lid that - when used in a rear engined car - is not accessible, because installed logitudinally it is on the front end of the engine. It makes it difficult to shift oil.
By the way - Abarth produced more than 50.000 engines for the Autobianchi 112 Abarths. So, there should be quite some parts out there.
So, to put it short - it is quite a task to alter a pushrod engine.
Give me time and I can find you a very comprehensive list (if I can find it) a German gave me years ago.
Best regards - and good luck with it.