I am not a great fan of Land Rover V8 alternators. They are problematic, they are very expensive, they have a weak output and they are hard to find.

The ideal choice for a replacement must vary from place to place as the decsion must be based on what is the easiest to find where you morgan about. Everything fails eventually (even exotic "upgraded stuff) so why ruin a weekend or much more wiating for something you cannot find on a Sunday at any garage near you?

In North America, the best choice would be a GM configuration..available everywhere in many aftermarket brands...even in chrome. They are metal rather than plastic cased and yet are lighter and smaller than the Lucas originals. A chrome GM costs $100 for 100 amps. An LR replacement cost $400.

To install you must fashion a small bracket to attach to the Morgan bracket. That allows you to swtch back whenever you wish. The images below should be self-explanatory. The WATCHPOINT is to make sure you shim or place the bracket so that the fan belt is perfectly aligned. I left a little space and shimmed with washers to suit.
 

I am a deep believer in sourcing parts that can be easily replaced wherever one is mogging. Ergo, in other countries, other alternators should be chosen. For example, in Europe, Bosch is the best choice to replace the Rover..