What about using a center punch and punch dead center of the shaft. Then drill down into the shaft, and open it up to the proper size to tap threads for a bolt to hold the wheel on? You could use a bunch of washers or make a sleeve out of a piece of pipe to fit under the washer and bolt to press against the center of the wheel. The splines hold the wheel while turning, the bolt would just have to keep the wheel from sliding off. I don't remember how large the shaft is, but a 5/16-18 bolt should be good enough. 3/8-16 would be even better if you still had plenty of meat left in the shaft.