If it is just the welded on U Joint, can't you take it to a good machine shop and have them very carefully cut it off? They could then reshape the shaft, mill out anything that needs milling, or simply make a new output shaft so that you can properly attach a new U Joint.
I recently took a mechanical box cutter to my friendly neighborhood machine shop, the cutter is circa 1965 and the company no longer exists. I now have a newly milled shaft, a new motor (from Grainger) and they replaced simple brass bushings with Timkin bearings so I won't have to replace the bushings every couple of months. A good machine shop can pretty much do anything . . . for a price.