I am surprised that you have had that many problems. I have had mine for 64 years. I ran it a lot as a child. It still works flawless.
During my days of buying and selling used trains I've had 19 of the 671, 681, and 682 engines. I only had one problem and that was this year. The wheels in a 682 I bought were frozen solid. After removing the motor, the wheels spun freely. With great effort I finally got the armature out of the motor housing. The grease had hardened like epoxy. After cleaning the shaft and the housing it worked great.
I agree with Eddie G. Unless it's sentimental, I would just buy another one. I would sell yours for parts if possible. The one I have from childhood was/is a workhorse. Looks like one on the bay went for $157 LN/OB
If I was buying and wanted to stay with postwar, I would get the 681 with magnatraction, or the 682 which has magnatraction and a extra piece of drive linkage.
Otherwise I would get the Lionel 6200 which is a remake of the 682 and has a really nice dark green paint job. These are easy to pick up brand new in the OB.
