I'll admit to knowing nothing specific about this engine, but here are some generic and fairly universal troubleshooting ideas..
Firstly, I take it that this is a 4- or 6-cyl engine? Does it have 2 or 3 separate spark coils, with #2 and #4 sharing one coil?
If yes to both, then it seems your issue is isolated to the non-firing coils (only one coil in the group is firing). Trace the wiring from the coils back to the ignition unit / magneto, and see if anything is awry. Likewise, start making electrical measurements of the primary & secondary resistances of all coils so you can compare them.
If you have an oscilloscope (one of the Fluke scope-meter type would be perfect) watch & compare the primary waveforms on all of the coils as the motor runs. Each should look roughly identical, a regular series of repeating pulses that increases in frequency in direct response to increases in engine speed, and vice-versa for decreases in speed.
Without a scope, you could also try an analog VOM for this - it might just be able to give you an averaged / integrated reading that rises & falls with changes in speed. Again, each coil should behave identically.
Meantime, maybe someone who actually knows this motor will show up.. ;-)
