Is that an electric start (starter motor on engine) or just a pullrope start? Whatever it is, a friend would be very useful, especially if pullrope. Friend can yank on rope or spin the starter, so you can be up close and personal with the spark plug. Hard to see that little spark from the core to the electrode, especially in daylight or bright lights. Hold the plug good and tight to the motor casing like Andy referred to, get good metal-to-metal contact (plug-to-casing), look up close for that spark.
If no spark, pull the plug wire off and toss the plug over to where you clean parts. Put at least a medium size screwdriver into the plug wire boot, making sure it's up there in contact with the metal clip. Hold the driver close to the casing, see if a spark jumps to the casing when starter or rope spins motor. A way that has worked well for me is to put a few turns of electrical tape around the driver shaft to use as a spacer. Hold the taped section to the casing, see if a spark jumps somewhere around the narrowest gap.
If you get a spark, clean that plug real good and try it again like you did before. If no spark from the plug still, replace the plug.
When done with all that, see if that green wire is attached to bare metal at one end. If so, it's likely a bridge from the motor to the kart frame. Not always necessary due to motor mounting bolts, but always a good idea.
Keep us posted, lots of folks enjoy helping out from here.