Titanium frame crack
Unfortunately, the best thing to do is to change the frame.
As the builder is no longer in the biz, I guess it will be more difficult to find someone who can fix your frame properly.  

You can still ride your bike even with the crack, BUT, the frame is now fragile because of this crack. The crack can grow bigger and bigger with vibrations and at some point, the frame will break on an impact. So you better avoid this case by changing your frame.