Well, thanks to Guzzi2go i was able to attack and repair my veglia tach too. Here are a few notes and pics...
Cutting it open took all my courage but was the easy part.
Once open and looking at the guts i figured the "hair" wires in the coil windings were probably broken. The 2 connections on the black wires on the circuit board showed no continuity... ends up it should read around 56.5 ohms.
I unsoldered them (they looked like the easiest to get to), unhooked the 3 connectors and turned the 2 plastic clips (yellow and brown you see there in the middle) to get the IC board out of the way.
I then seperated the shaft assembly from the base.
DISCLAIMER: To get it all apart and get the coil out I ended up breaking the tiny end of the shaft that is held by a small copper ring pressed onto it. It prevents the assembly from pulling out upwards from the base. Once reassembled this did not seem fatal - but is clearly not ideal. Not easy to get around in my opinion.
I bended the tiny clips aside and got the coil free to find the broken wire underneath. I unwraped it just enough to resolder it in place (don't forget to burn off the varnish from the wire with a lighter to get a good contact).
Reassembled it : a bit of a delicate PITA - but it now works!
I applied silicone to the casing to seal and hold it, but had to remove the outer bead to make it fit in the instrument panel hole so i added a wrap of duct tape too.
Overall i'm happy it now works and cost me 0$, but the seal between the gauge and lower plastic cover is not as tight as it was... Notice the gap between the 2 halfs in the pic - i wanted to account for thickness of cut but that was not the best. Maybe would be better to rivet them in place with a back-plate or something. Oh well, next time
Overall it's fixed!
Hopefully this helps the next guy.