Unless you've added a killer stereo and enough spot lights to illuminate an Air Force Base, an alternator upgrade isn't really necessary. Besides, the Lucas C40 generator is almost as common an item as one can find in the British Car World.
I had the original generator for my Herald (1962 1200 model, Generator date code of 11 61) rebuilt for about $90 a couple years back. If yours proves to be unrebuildable (unlikely unless there was major "fry" damage), find a rebuilt one. You might even be able to obtain one locally through a regular auto parts store...just say Spitfire if they look at you funny when you say Herald.
There are "new" replica generators out there. So far, everything I've heard about them consists of "they're very expensive" and "they're not really all that good"!
Meanwhile, congratulations for "trying to keep up with [that] tr6"!! /ubbthreads/images/graemlins/cheers.gif