I had the same problem with a SCX Audi DTM car I bought used (Probably why it was used) It was super slow and the motor would get very hot along with the smoke. Same results with the motor out of the car on my bench power supply. I tried the same things as you did but to no avail. Decided to take the motor apart because back in the 90's I raced a lot of 1/24th and worked on lot of motors. After checking out the armature for shorts and such the only thing I could figure out was it seemed like I could hear the arm scratching something when I turned it by hand in the can with no brushes. Carefully looking at the armature it looked like one of the laminations had shifted. I chucked the arm up in my lathe, took a couple of light passes to even out the stacks. Put the motor back together, ran it on the power supply and it seems to work ok now. Right now I don't have a running track to fully test the car but think it will work, if not I'll just adapt a different motor in it.