When you say you checked the timing, how did you do that if you can't get the engine to start? Did you mean the valve timing or the ignition timing? Although, you did not mention whether or not the it will start with the electric start. Does that work? Does it ever jerk the pull starter back out of your hand when it backfires? I assume you have spark if it is backfiring. Have you checked the flywheel timing key? If you take off the stator cover, you can look in and see if the key has sheared off. That would be a shame because that is like a $2 or $3 part, and that would mean you have wasted all that money on a whole new ignition. Have you checked the valve lash?
Yours truly, sincerelly, love always, Josh aka The DirtDemon.