Dennis is correct about the GP 25 backplate. In fact, I have a GP 25 that was worn out — bought very cheap so I'm not complaining — that would run in cold weather and not when hot.
I replaced the piston and cylinder with LA 25 parts and it works just fine.
To backplates, there is somebody on the forum who NC machines batches of backplates to suit the FP 40 and FP 25 from time to time.