[Evan Austin]

Hey Sanddrag,
If the shaft is flared on the end from hitting it even a little bit, a gear puller may not be enough to pull the sprocket off. Think of it this way, if the shaft gets flared out ten thousandths of an inch, that will be like trying to press together (or in this case, apart) an extremely hard press fit. Check the shaft with calipers to be sure. If it is flared, fix the end with a good file or a bench grinder.
My only suggestion for loosening the sprocket would have been to heat the shaft and sprocket hub area, then spray with penetrating oil while it is still hot. I know it sounds a little dumb to spray light oil on very hot metal but it has always worked for me and never yet caused a fire (emphasis on YET). Good luck with the go kart, that's gonna be fun!