I have no idea if the actual horn units are the same, but I would suspect they probably are. In any case, I would think pretty much any horn could be fitted (although may need alteration to the wiring connector).
Is your 45 a Mk1 or Mk2/facelift??
If the latter, it is more likely that it is the relay within the Pektron SCU which has failed. They are not too difficult to replace if you have a soldering iron and a desolder pump or braid.