Try David Woods at Chronometrics
Hi,
Have you thought about trying David Woods who operates as Chronometrics, advertises in MPH and is a VOC member. He knows everything there is to know about these.
I would recommend you think about trying him for a fully reconditioned unit.
If it is the head gearbox then I understand that there were two types, early and late so you'll need to identify to David which one you have.
As relating to the drive in the front wheel, I have seen loads of what I thought to be the same thing at autojumbles only to then find small differences, different spacings, different drive shaft finishes etc. so maybe the answer would be either have a part number (can't see one on mine) or take a Vincent one with you when you go looking.
The front wheel speedo drive, I stripped mine, cleaned and overhauled it myself and put it back together again it worked fine.
The head end, I've had stripped gears and David sorted those out for me for about £35.
Hope that helps.
Stuart