I don't know what your opinion is about originality of those special cars but as Carl mentioned before, and certainly other people think , please stay with the original veloce parts if possible, it won’t be cheaper but you won’t regret it at the end.
If you need parts or even a complete MT40B, you will find it on the net with some patience and when looking with the eyes open.
Yesterday, I saw the correct and complete Marelli on Ebay . "In working condition" said the seller but pricy as they all are . Please note I have no connection with that seller !
Pay attention not to buy the MT40A, it has the same appearance but that starter is for the Lancia Appia and can’t be mounted on the Giulietta although some parts are the same on both types. Often sellers don’t know (or pretend not to know) and advertise their MT40A as a Giulietta starter.
xalfaracer is correct too:
Marelli’s are lighter, … another reason to keep the Marelli on a 'Veloce' car!
Weight Marelli MT40B : 3800 gram
Weight Lucas 25301A : 5170 gram
If you would share more specific information about the story of your starter maybe the raison of the problem could be found, who knows, on Alfabb much is possible?