Only if the change out the stator (and or rotor) or rewire it as well.
To tell if it is oem to your car look at the rear bearing, as I understand the cap should not be flat it should have some round ridges on it, the diodes have particular markings (Motorola????)
There are also date markings on the front and rear case halves. Should be days to a few months before your cars build date.
Lastly if it is not a rebuild don't let them sand blast it (if you are looking for oe restoration) I don't recall but there are cleaning methods that will not alter the casting patenia.
Also sometimes the long bolts get replaced and don't have the correct heads
