Many thanks for all the comments!
Auch für die Deutschen!
I know and use the site thruxtonforum.de for a couple of time now....
Anyway: the used speedo has been bought by a german manufacturer "MMB Messtechnik". Its tiny, looks great, comes with a hall sensor (optional) and works really fine for months now.
I had to lathe an adaptor ring (alloy, polished) to fit in to the dashboard (where the original rev gauge was mounted).
And it was cheap: 115,- € (which has to be calculated in US$ by yourself!!
).
One pic of the speedo (not finally wired).
