Vintage Voltage Regulator MM

British cars from the 50's and early 60's used very similar regulators. I would take a look at the repair manuals for a 50's Bentley R type and an early 60's 3.8 Jaguar E type. I believe both manuals have procedures in them for trouble shooting them. Years ago I found the Bentley manual on line. Old Rolls Royce and Bentley manuals tend to have very detailed manuals with a lot more info in them than most manuals. I would also take a look at some British car forums for other cars including Triumph TR4 and MGA's, these cars also have mechanical regulators and the owners sometimes write very detailed troubleshooting and repair guides.

 

you may look at a bosch dealer, the old bosch regulators looks equal

 

Those are resistors on the bottom of the voltage regulator. Tim at Select Electrics can test and repair the regulator: http://www.selectelectrics.net/home
If it is unrepairable, then you should be able to find a good used or new replacement. The IR19B is fairly common, a vendor in Padova had NOS units in the box.
They will cost you, but we are talking about a 1/2 million dollar car.

 

You can call Norbert at GT Classics in Long Beach, he may know someone who can re-build it without having to put some non original part in your Ferrari.