Gunner Asch fired this volley in
:

I've looked the engine over..and I honestly cant find a place to put
the sensor. And its a very very good idea!

There's a reliable tach sensor on EVERY points-n-condensor motor.

The points. That's how all old-style tachs worked.

Another simple solution is an inductive pickup coil around any single
spark plug wire.

LLoyd