The on/off button

Try making one out of hot glue, a bit of plastic or rubber, or 3D printing. To make a button out of glue or plastic, remove the logic board (you won't have to remove the battery as it sits in a cutout). Then trim the new button or add glue as needed until the logic board fits back in neatly.

My hot glue button looks like this:

It sits quite stiff in the button hole and I can use it by pressing it firmly with my thumbnail. Incorporating plastic from a recycled container would probably make the button less stiff if you prefer.