It is not really an easy fix.
The galvanometer is a piece of watchmaking, way more delicate than the Compur shutter - and the galvanometer is matched to the selenium cell.
So, in case somebody approached me with this, I'd tell him to live with it, and in case he insisted on having it fixed, we talk about finding a good match galvanometer or a complete set (hard to find, expensive), or stretch some plastic sprue over a candle, cut off the 2-3 missing mm and crazy glue them to the existing needle (hair splitting technology) and hope the weight will be perfect - or drill out the old needle and replace it with stretched plastic sprue.
https://www.youtube.com/watch?v=pw0fv6Ot7eQ