Very useable. Rough up the surface and put them back, BUT they will probably contain asbestos so be very careful and remove any dust immediately from the work bench and tools, without making anything airborne. Definitely no power tools.
Can't believe your considering reusing 54 year old break linings, do yourself a favour and bin um before you or anyone else breathes in any of the asbestos contained within them.
Look at getting some decent replacements from the likes of EBC, Ferodo or Newfren. £20 odd quid at most..
The asbestos bit is a worry but tend to reuse them and as much original stuff as possible. The newer brake shoes seem better and I have used some but at one time the new ones were crap or the ones I bought were. Funny things brakes on Lambrettas they can all perform differently, good and bad.