Normally closed SSR won't work with grounding out a magneto's primary to ground
Once the SSR has been in the magneto circuit does it still work with the light bulb?  Or have you fried the SSR?  (you could just measure on resistance at low voltage.)

As Enemy of SM hinted.  I suspect the high voltage from the magneto has killed your SSR.   There are likely to be kilo-volts on the line.