The real question is how much you want to spend to repair it? If you could find a plain spline hub or coupling that matched the spline shaft, a bit of machining might get a replacement insert that could be brazed in place. This assumes the hollow shaft where the old spline was is steel and not aluminum. The old hollow shaft would need to be accurately bored and the splined hub/coupling machined for a close fit and then brazed or welded. If you don't have a lathe, find a friend who does and likes to tinker.