Start with a known good or new spark plug. Check any & all safety switches, wires & connectors and their terminals. Take a secondary coil resistance test, trim back the plug wires 1/4" on the plug end, check pick up coil cranking output and see what issues, if any, come up. After that you are looking at tracing the issue back to the cdi box.
The stator does not provide spark, so that can not cause a no spark condition. The pick up coil reads off the stator flywheel, but not the stator.
Check the p/up coil output for 12vdc while cranking, & trim back the plug wire(s). If the pick up coil does not produce a 12v pulse when cranking it is bad. Unplug the pick up coil, the wires that come out with the stator wires are the p/up wires, hook up a meter & crank the bike, you need 12 volts dc from the p/up coil. Anything less than 12 is bad. Out of the left side engine cover you have three yellow wires to the regulator those are your stator wires to check once unplugged from the regulator, that also has two wires going to the cdi. The other 4 wires are for the two pick up coils. Check those for power when cranking.
If you have a weak battery because of a failed stator or regulator than you can have a no spark condition.
Thanks for your quick reply....Plug has been replaced with a brand new one. If the stator does not cause a no spark issue then that should not be a problem. ....this is basically the same answer from a similar question another user had on a 1998 model which left me wondering a few things
what/where is the stator, and what / where is the pick up coil....i dont see any of these items on my electrical parts diagram? (do i pull off the side engine cover to test these items for current) is the stator and regulator the same thing as i do have a rectifier and regulator assembly on my parts diagram. http://www.cheapcycleparts.com/model_years/4574-yamaha-1990-big-bear-YFM350ERA/assemblies/131833