Taku, I recently came into owning a Bachmann 4-8-4 Northern w/56'tender.  This was a Burlington route loco, still in the box, wrapped in the original plastic.  Based in the info that I received, it had been in storage since the early 80's, and had never been ran. 
This was a gold-find for my young modeler son as we grew up along the mississippi river in Burlington, and having traveled the route to Quincy, Il many times. Sorry the history.
I would be interested to know if you have heard anything back from the Bachmann company on your 4-8-4 repair.  I recently contacted Bachmann and was led to believe they no longer make 4-8-4 in a HO scale.  Are replacement part available?
thanks,