You removed it too soon, but if you have it apart on the drive sprocket side put a socket over the nut that holds it on (more often than not the sparkplug tool that came with the kit will fit this, put a screwdriver through the holes to get leverage) and turn it to the right (not unscrewing it just yet) and watch what happens on the other side in the clutch area.
If neither gear turns then the large gear woodruff key went, if both spin but not the piston then it's the small gear key (most often offender.)
The gear is removed with the puller tool, key replaced.
It's recommended a very thin-walled washer is used to fill the gap that the locking ring doesn't always fill. The only person I know who has these is bicycle_motor_works on ebay, it's a small thin washer made of copper, and fits around the end of the crankshaft arm but inside the gear smaller than the diameter of the threads inside that lip. This way the gear is firmly pressed onto the driveshaft rather than being only almost tight when the head of the bolt bottoms out on the end of the shaft, premature of the correct press fit. (that's what she said)
I made one in an emergency situation (it was that or walk 4 miles or even worse take the bus) once, I used a multitool to cut an old keyring I had on my swash of keys into an equally small ring that fit perfect in that spot. Alas the damage was too much, and my crankshaft end had already been worn down to a mear couple millimeters too small...
Well anyway if I had some soda cans I could have made a really janky cap to fill in the difference, now that I have copper sheeting on the bike as just a large curved copper fender I can simply sacrifice a piece of that rather easily and actually "make do."
Anyway it's a problem that can be solved before it began. Probably why that specific gear has had so many problems with the woodruff key. If you just aren't aware that that gear isn't perfectly down tight from factory then it can end up being looser than normal, not good.
Then there's the too long bolt in the too small hole situation.. The end of the bolt bottoms out in the bottom of the threads but is too long for the head to actually press down.
If you replace the bolt or want to improve it then make it shorter than the hole is deep, and put a small washer in that area, I could even assume a piece of heavy gauge solid copper wire trimmed to fit in that small circle would work. Either way filling the gap and then torquing it down will lead to better results keeping the gear firmly in place while not adding any extra particular burden to maintain the motor.
