I completely agree with Chris's statements. If your dynamo works, why change?
We do not drive modern cars, with all their electrical components, and current drain!
And it is not frequently you drive your Aurelia with all electricals on...
However, to answer the OP, Aurelias do not mount Bosch dynamos, but Marelli.
The correct reference for the dynamo for a B12, taken from the parts book, is Marelli DN 22C-200/12/1700D
The parts book also lists all dynamo components, including the bearings!
(RIV 01A and RIV 2065b, if you need them)
And the required regulator, also difficult to find, is Marelli IR 15A
The initially mentionned MRG 90/6 1500 L51 is clearly not an Aurelia dynamo, it is fitted with its regulator, and is 90W / 6 V I assume.