Speedy Cables will make one up for you. Phone or send then the original. I suggest that you check to see if the inner is binding or whether the ends have become rounded, not square and therefore slipping. Also check whether the ends have become loose. Finally, on 1970s 4/4s! the gearbox end of the speedo drive had a right-angled drive, possibly a Lucas or Wipac motorcycle part similar to those fitted to the front brake drum of Vincents.
Google 'Speedy Cables' for their details. Good luck!
