Install a tiny tach Look for the Commercial powered one model DTTNR
Several members here have it on their tractor and it works well for them.
$65.00 and requires a tranducer mounted "around" the fuel line.
The tiny tach works well, and comes with the transducer to clamp on one of the injector lines, just give them the diameter of the line and they'll send the unit and all the parts with instructions, it's really easy to install.