RE: Saito FG57 Fourstroke
If you test it with a magnet you hold in your hand and it works, then, as mentioned above, it might still not work on the engine, because of the polarity issue.
If you have the RcExcel style sensor in post 44, the one with 3 wire futaba plug, the sensor might have been put into the plastic holder, reversed.
It is in fact possible to get the sensor pulled out of the plastic holder, turn it 180 degrees, and put it in again.