Personally I never had any good with JB weld on anything other than no stress items. I see way this going to work since it a lot weaker than the metal that has already been worn. I even tried a product by Prematex call Steel Weld epoxy with terrible threading results. It would have never held mounting blade spindle.
If you can't get a replacement shaft then welding would the best option in my opinion; just make sure to center the journal on the shaft for balance.
Question though as why journal is worn so far in the place. Was it one those journals the roller bearing fitted on loosely? If so then the new shaft would do it again; unless, the journal is knurled so bearing has slight press fit. If is instead a bushing type bearing then the journal area need to near perfect smooth to prevent wear.
