There are quite a few options available. Since there is no spark with a diesel, the standard tachs will not work. There are several ways to get a tach signal on a diesel including injector lines, alternator, VR sensor, optical sensor and even mechanical hookups.
One of the more intriguing ones is this -
www.youtube.com/watch?v=HhLPAPZxNsM
Do a Google search on "
diesel tachometer" and you'll find a bunch of others one or more might be viable options for your application.