Tiny Tach is what you need if you really must have one. I don't have one, but I have not missed it. I can use Hz from my multi-meter if I am using the PTO generator, and I have an optical tach to check things like RPM when needed.
I put a Tiny Tach on 10 years ago, it still works. BUT I never look at it, run the tractor wide open for mowing, running the chipper, snow blower, tiller, spreader. I run the FEL work at 3/4 open or so. Believe me you will hear what you need as far RPM goes. Diesel engines love to work at WOT.