does the engine turn counterclockwise from the direction shown above? and does that clutch nut have right hand or left hand threads? if it turns counterclockwise and the threads on the crank are right hand, it's as simple as a standard prop adapter like I've made many of... if left hand threads it's doable but requires pulling the crankshaft and drilling and tapping for a stud. If it turns clockwise from the picture shown you would use the other side and do the same thing.

about spark, you've tried a couple different plugs? set the coil gap with a business card? just checking, it being a stand alone magneto should get spark above a certain rpm...