We have a wealth of parts here to keep your old alternator going:
https://www.dmgrs.co.uk/products/rov...-and-yle102500
Usually you can get away with just the brush pack / regulator (usual cause is worn brushes or diode failure, both of which are part of the regulator) but occasionally the slip rings also need changing.
It's a very common issue on the diesels - exacerbated by oil being dripped on the alternator when changing the oil filter.
Always cover the alternator with a rag before removing the filter.