This may be of interest, there used to be a toolmaker in Verwood who, as a sideline offered a service removing stubbern
studs, bearings etc from vintage motor cycle engines.
They used Spark Erosion machines to good effect & in Jon's case this would leave a ring of very thin metal which could then be easily prised out.
It was a very expensive way of doing it, a new engine would have been cheaper. It was only used when the casting or machined job had a lot of cost in it
