When did it start running bad. Right when you got it back together? or well after break-in? How are you telling it has a weak spark? It was hard for me to test when having a firing issue with an old '85 125, I worked on. The best way I found was to remove plug, put it into the wire, and kick. The compression kept me from kicking fast enough to get a good spark. The spark was never bright and scary like a car's spark, either. I also read, you should always have a plug in it when testing. Without the resistance of the plug, you can damage the CDI. Also, sometimes a weak spark is a result of a weak ground connection. You might remove the Cdi, or coil, clean mounting area and re-install. The reason I asked about when it started messin' up, is that when a motor is getting worn, it usually needs fattened up with jetting. After a fresh top-end, it would be rich. You say plug looks good, so that may not be it, but how does plug reading get affected by weak spark? Where is it blubbering? All the time? Bottom rpm range, mid, top? Have you inspected the reeds?
Don46 would be a great guy to ask. He has reverse Alzheimers and can rattle off a bunch of spec.s on that kinda stuff. I guess he has trouble with yesterday, but is like an owner's manual for any bike he has owned.