What year is the Disco? I have an 80 amp unit on mine, aparently they wer export spec and I've only ever seen one other, I even asked at the reconditioners.
Try something like a Lucas unit from a newer Ford in the scrappy but be careful, my 80 amp runs from a single Vee belt and it can shred that belt in a few hours when the twin fans are running, it really needs a multi vee belt.
The later SE and LSE RRC had a 100 amp unit running from a multi Vee belt direct from the crank, if you have a non-serpentine V8 you could retro fit this to it. For a Diesel I don't know the answer, but the 100 amp alternators are cheap enough on flea bay.