Lister BRUSH 15KVA RS2 Wiring diagram/ repair

[Lister123]

Hi All,
Looking for some help regarding finding a wiring diagram or any Info or advice on how to repair following generator. Has been working fine until a couple weeks ago, now not making 230V only making 2/3V I have tried flashing the excitation with no luck and when 12v battery connected across excitation generator is producing 80v. Looking to repair generator and/ or Modernize components if parts obsolete. Please see attached photos. Any help or guidance would be appreciated as this is a back up generator for remote area which is prone to power cuts . Thanks

[Lister123]

Attached data plate photo