The main internals are likely to be standard stuff used on most cars of the period. Only the casings which give the mounting lugs might be unique to particular applications.
First you need to identify the manufacturer, likely to be Magnetti Marelli. With that Google may well find all sorts of bits for it. If you can identify the 'family' of internals, there may be alternators about that coould
donate their internal organs.
Some manufacturers include detailed dimesion drawings in their catalogues. A modern alternator may well fit with little or no modification to the mountings.
Also, do a search for auto electrical specialists, sadly less common than they used to be.