There are a couple of versions of tachometers that could work for you. One has a wire that vibrates at a given rate when it's on the rpm. Another is an optical tach. You put a marker on something like the pulley and it reads that. I've seen reference to another that clips over the sparkplug wire but never used one other than on cars.

 

I used a digital laser tach on the pulley that operates the impeller. The marker I used was a piece of reflective tape that came with the kit when I bought it.