Remove spark plug...Plug into wire..Turn on ignition, SLOWLY rotate CCW, mark case and hub(draw a line across) when you hear spark...Measure case diameter at mark with a dial caliper, multiply by 3.14, divide by 360....Multiply by 28 for C&H, 2 for Falkon..Mark hub that far from first mark....Find TDC, mark crankcase..Move magnet ring until the distance you found is that far from TDC mark rotating crank CCW....Close enough for government work.. Or toy airplane engines...
Hardest part is moving the ring, the setscrews make burrs....