I would buy another handpiece (kinda pricey, I know, but how much is
your time worth?) and send the broken one to Gesswein for repair.
Sooner or later the new one will give up the ghost and when it does,
you will have a freshly repaired spare.
If you do that however, when you get it back and before you put it
into storage, make sure it works properly. I didn’t check out a
hammer handpiece I had repaired once, I just put it away for when I
would need it. A couple of years later when it came time to put it
into emergency service, it didn’t work. Not only was I without a
working tool with both of them broken, the repair warrantee had