i found an interesting webpage ( http://deskthority.net/wiki/Olivetti_dome_with_slider ) , explaining the keyboard technology and which olivetti models are using the same. I have an spare/broken M21 with a complete Keyboard, so i can take some from there and replace it in the M20.
The keyboards can be disassembled and then new piece
made on a 3D printer.
Pinning and gluing can also be done but still requires disassembling
the keyboard.
What drives do you have?
Dwight