When I bought the Tacho a good number of years ago, I was told it would suit a JP, there is no part number on the box - just marked Lister Tacho 0-1000. I added the part number from my manual. Despite exhaustive attempts, I was never able to source the rest of the drive parts to try it out.
Given that your engine is rated at 1200 rpm max (which I doubt you would ever achieve on the cut) what would you estimate the revs to be when you would be red lining the engine? My point being that 1000rpm max would probably suffice for a none loaded narrowboat. I will remove the part number from the ad.
PS - there is a number 50/6711 in white paint on the rear housing which I suspect is the MOD part number