I wonder if Tellspin's reference to a pulse switch is to the non-Hall sensor used on older 3W (and maybe other ) ignitions? If so, I would not bother with it and use a Hall sensor.

Tellspin: Your related question on the Open Source CDI thread will probably get you an actual part number. You need a unipolar, non-latching sensor. Although Hall sensors are basically a specialized transistor, you simply cannot make a practical one at home, and they are relatively inexpensive to buy in the USA.