There is a way to do it but you need to be handy with fine tip solder iron and understand how frequency to voltage converter ICs work.
Two years ago i went from Atomic 4 gas engine with Coil sensor based analog tachometer, to westerbeake 1-108 with magnetic pickup. My main reason for doing this besides $$$, was that i wanted to use my old instrument panel and diesel tachometers do not come in 2-1/8" diameter. I had no room for a 3" or 3.5" hole on my instrument panel.
What i ended up doing is taking the tachometer unit apart and modifying the circuit board to work with magnetic pickup based signal.
Then calibrating display unit under various RPMs using a separate RPM
meter. Most tachs have a potentiometer(calibration knob) in the back to use a fine screw driver and adjust the needle deflection.
Saved me few hundred dollars and more importantly, space.
Let me know if you're interested on how to do this.