What you need is an alternator/AC Tap/Stator tap driven alternator. They are often called "diesel" tachs.
There are a number of generic ones out there made by companies such as WEMA, Faria, Vee-Three, Stewart Warner etc. etc.....
I use the Faria's quite a bit as I find them easier to adjust and dial in than some others.. They also use the standard brass studs for wiring where WEMA does not. Vee-Three Instruments has taken over the Teleflex brand of instruments and I've not had very good luck since and have had numerous gauge fogging issues, even on brand new gauges..
I am using mostly Faria at this point because they are reliable, easy to install and their tech support is also good.. I am sure the WEMA gauges are good but when doing swap outs it is much easier, and less expensive for the customer, when the gauge uses industry standards such as the brass studs vs. a pigtail harness.
Normally to calibrate a diesel tach you divide your crank pulley by the alternator pulley diameter and then multiply by the number of poles in the alt. For a Motorola alt this is 12 poles.
This math will give you the initial "ratio" setting to dial in. You then make the fine adjustments with the adjustment pot and a digital photo tach until it is dialed in...
For example: 4.6" Crank / 2.4" Alt Pulley = 1.91 X 12 Poles = 23
This setting would indicated dial position #4 on a Faria tach. Once #4 has been set you can then do the fine tuning with the adjustment pot and a photo tach.
The Faria TD9102 is a 0-4000 RPM diesel tach with digital hour meter and Defender has them for about $119.00. They also have different RPM ranges and offer one to 6K IIRC..
Faria Euro Style Gauge Black With White Needle