I haven't actually finished the job yet but here's the information I've gathered so far:-
Help identifying diesel tachometer req'd - want to fit it to my car!
For a mistake purchase and then the correct one and some bits I got from funola...
For the larger bracket which is also the pointer for the timing marks see this before you take it off!
Timing marks gone from my OM617 engine
Just remove the sensor from the larger bracket (see first thread link).
It should work providing that you've got a little pin sticking out of the crankshaft balancer and you've got some power running to the tach amp (I'll probably get mine from the supply to the glow plug control box)