http://www.osengines.com/parts/images/29008219.jpg Not a way spring. I did not mention a spring. That is a Woodruff Key. I assumed that maybe perhaps because your chain got caught up you sheared that key? Are you familiar with the puller tool that removes aluminum crank arms? Your sprocket uses a similar tool to pull it off that probably came with your kit. If you remove the clutch cover then the nut and washer on the sprocket you can get a visual with a light to see if the key is sheared. Sorry I could not help some else will pipe in soon enough..
The sprocket uses this keyway as a safety feature not to tear more stuff up. The open notch inside the sprocket you see in the pict is where that key goes there will be a notch in the shaft that the sprocket is attached to that rymes with that keyway.
Now when you go to take the nut off the sprocket it is going to turn the motor. That is why I say hold the bike still with the chain still attached. Other wise you will have to wedge something against the sprocket to hold it still that could get sloppy. Ether way it can be done. Turn the nut counter clock wise to take it off.
This is a video that shows use of such a puller.
http://bicycletutor.com/replace-cotterless-cranks/