Not sure what facilities you have but I would make up a 1/4 inch thick washer maybe 2 inches in diameter to slip over the shaft so that when you weld the axle back together after beveling the end of the axle so you can get plenty of weld inside. Then grind the weld back so the washer can slip over the shaft and butt up hard against the sprocket. Then you can weld the washer to the sprocket and also to the axle. Just make sure before you fit the washer you will still have clearance between the bearing and washer. Just take it slowly so you don't get too much heat distortion
