You could try swapping the speedo clocks round, I did this on my set and it was dead easy, unless the speedo shape/screw fitment changed between motometer and vdo? This way you know it will definitely work with your speedo cable and you keep your milage.
All you do is:
1. Remove the 8 screws/bolts and bulbs from the back of the clocks
2. Carefully pull the circuit board and clocks back and down
3. Unscrew the small ribbon cable from the back of the speedo and extract it
4. Unplug the ribbon cable that goes through the speedo into that 'L' shaped circuit board clipped around the speedo
5. Remove the 'L' shaped circuit board, some are held in with a screw some just clips
6. Swap the circuit boards over so the same board is used with the same clocks
7. Install the speedo and reassemble your clocks and you're done!
There is a more detailed description on how to dismantle the clocks Here:
http://www.4130-products.com/step/install/index.htm, but you only need to remove the 8 main bolts and not the nuts. I could take some pics if you like!
Of course all that is useless if the motometer speedo wont fit in the vdo cluster and/or your old speedo is busted!
