The speedo in my 1986 IM 6 looks like a 914 tach but I'm not sure, 150 mph. It runs off a cable that goes to the driver's side front wheel...the same set up as an old VW. Its not accurate but I haven't bothered getting it recalibrated, just used a GPS. The tack looks to be the same but its red lined at 6,500. It runs off my new ignition system so is probably from after 1977.
I changed my ignition system to a crank fired Electromotive system, so I do have all the old parts such as distributor, CD box, coil, etc if you are interested. You might also consider using a crank fired system. I picked up mine used off Ebay or Samba for a very reasonable price and have been very happy with it. Wouldn't go back to a distributor unless I had to.