Hi
Yes you can mount another regulator inside the original case . But , you need to know if your generator is fitted with Type A or B field coils
A. Internal grounded field coils
B. External gronded field coils
I used to know from memory, which is which.. ford, chev, Jeep, Dodge , Lucas etc.
The vernco web site explains it all very clearly. In many cases, you can bring them back to life with a clean up and adjustement of the points . The points do wear and suffer from arcing .The resistance bars are switched in and out of the field coil circuit by the vibrating points.
Here is the vernco site
http://www.vernco.com/sparks/id569.htm
I recall the days at Camberwell, where Keefy worked , NOS ones in the box were $5 each ...
Mike