Faria has a rebuild service, reasonable prices, decent turn-around times... and if your hour-meter is incorporated, you'll lose that.
Assuming you have something like the analog Chesapeake models, they'll be semi-accurate at only one RPM level from idle to WOT... wherever you choose to calibrate with an IR tach... so at idle and WOT, not so much.
I'd say your better off replacing both tachs with the Aetna Engineering digital tachs. In our installation, it was a drop-in replacement... and now I actually know what RPMs the engines are making.
If the hour-meter is part of the equation, I'd suggest it's also better replacing both tachs at once... so even though you won't be displaying actual engine hours, at least you'll be displaying similar relative engine hours for each motor.
Not meaning to dis Faria... I just wasn't happy with the inaccuracy... and could have saved myself some steps going immediately to the digitals.
South River, Chesapeake Bay