I have restored several of the English Duo-Art electric players over the
years.  I have always simply earthed (grounded) the switch casing, along
with the motor frame, to the earth conductor of the new three-core mains
cables I always fit them up with.  That way there is no danger of shock,
you keep the instrument original and should there be any electrical
leakage it will blow the trip or fuse in your house panel.  I have never
had one do that, though.

David Evans