Could it be as simple you don't have the right address for the decoder? If you have not changed the stock address it should be 3. You could try resitting CV 8 to 8 so the decoder is at factory default and address of 3.
I hate Rust
When you reassembled the mechanism, are you sure you got all of the insulating spacers between the weight halves and that the motor brushes are totally insulated from the frame halves ? From your description it sounds like one of the motor brushes may be making contact with the frame or there is binding in the gears because of improper reassembly.