Roger
Many of the electromagnetic clutches used on machine tools had adjustments to set the plate gaps for proper operation. Infrequently needed so by the time it was required the book would have been long lost so folk guessed. Which rarely ended well. Generally finding the proper procedure and following it sorts them. But the plates may need a good clean first.
Coils have been known to burn out after many. many years. If so you are probably stuffed as most makers have either died or pulled out of the market. Coils are invariably encapsulated and more or less "everything proof".
Retrofitting with a modern device may be possible as may be coming up with a mechanical work around.
Clive