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Most likely there is a tapered shaft not unlike your prop shaft, with a key that takes all the force of steering. The top of the shaft would most likely be threaded and a nut would be affixed to secure the wheel to the shaft. From what I see in your post, the nut and top of the shaft may be totally corroded, and if this is the case, the wheel may be fused to the shaft. I would first grind the corroded part down to where I could see the profile of the shaft and inner part of the wheel. Then if there is a key there, you could tap on it and put some solvent in. I am not sure a flame would work because it may damage the wheel, but heating things generally makes them move a bit and it would promote seepage of the solvent (something like Liquid Wrench product). Once the solvent/lubrication has had a chance to work, I would try a wheel puller and you may have to drill a dimple in the end of the shaft to keep things steady. Beware, a wheel puller may deform the wheel if too much pressure is applied. In the end if you are really determined, you may have to literally drill out the insides of the shaft to the point where things finally release. If you cut the shaft off and take the wheel to a machine shop, they would be able to press the old shaft out without hurting the wheel. If you cut the shaft off and drill a hole into the remaining part of the shaft big enough to insert a saw blade, you can cut laterally toward the wheel hub and this will help make the shaft lose corrosion bond with the wheel. Hope this helps, that is the mindset I would take if that wheel is really something to save, and it sounds like it is. Be gentle as there appear to be several ways to damage the wheel.