If you have weak or no spark you may want to first replace the spark plug to make sure that it's not causing this problem. If the spark plug is good then the ignition coil is most likely the cause of the problem.
You can click the link I've attached below and use the engine model number to pull up a parts diagram so the ignition coil can be ordered.
To replace the coil: Remove the engine shroud (starter side of the engine). Once the shroud is removed you should be looking at the flywheel. On the side of the flywheel you will see the ignition coil bolted to the block with 2 screws. Remove those screws and pull the coil away from the engine and then unplug the little "kill" wire that's plugged into the coil. Put the new coil in place (plug in the kill wire) and adjust the air gap between the coil and flywheel magnet to about .010". Put the shroud back in place and you should be back in business with the generator.
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