It looks like you are capable of doing the work, so really what you need is a resource to talk to or face time with who is familiar with Mangusta hubs.
I don't have specific experience on the Mangusta, but the length of the spacer between the bearings is critical as the inner races are compressed against it. Then it should be fairly straightforward axle, inner flange, spacer and nut.
An old thread here:
https://pantera.infopop.cc/top...-bearing-replacement.
Try reaching out to Steve Liebenow (username Mangusta), I don't know a way to tag a user on this forum.
PS: Should anyone need there's an eBay seller 'Italiomiami' who has uprights listed on eBay!