Did you try the MB Classic Center? Also the big MB salvage yards, like Potomac, or Silver Star?

You can certainly weld this back up and machine it to size. I worked in a small machine shop/job shop where we did this kind of thing all the time, mostly on construction and mining and oilfield equipment where the price of a new shaft was $$$$ yikes. To restore the original strength, the shaft would have to be re-heat treated: annealed first and then re-hardened back to spec. It's been awhile, but if I remember correctly, what we did was usually the old shaft was annealed first to soften it and make it more dimensionally stable to minimize distortion during welding. Then shaft was returned to machine shop for weld build-up and machine back to finish size. When complete, then back to the heat treat shop to re-harden. Take it to your heat treat shop first, have them measure the hardness and determine if it is case-hardened or through-hardened, then he knows what to restore the hardness to after the repair is done. Cost of the heat treat I would guess to be $250-ish, welding and machine work probably four hours or so, assuming no keyways or splines are being cut, so there's another $400 or so, so that gives you a really rough idea of cost. For a keyway add another hour and splines add another 2-3 hours.