Ok, first off, I noticed a very heavy brown grease or oil on the gears when I blew the picture up, thats a no no, which could lead it to dragging on the motor. As I suspected that it had and Athearn BB type motor. So what you can do is buy a replacement motor and reuse your old flywheels. What I would do at this point is to pull the motor and worm gears and see how it rolls on the track, if it seems to be tight or dragging you need to take apart the trucks and clean all the grease and oil off all the gears and center guide points. Then put the trucks back together and give each gear one tiny drop of Labelle Oil on the guide pins. Then after they are assembled free roll them and make sure they roll free. You can inspect each gear also to see if there is any burrs they are catching on. It is important to make sure the trucks roll smooth with no dragging or a catch in them, and not to over lube them.
After you have done the trucks, look at the motor and see if the windings have a white looking film on the winding, and smells burnt. The white dull color is from the windings being overheated which will make the motor “weak”, or you may notice them being almost black witch mean it got really hot at one time or another. If all looks ok and the motor spinns free with no dragging, put it back in and connect the drive shafts, and give the engine a run. If the motor is weak, it will not change by doing all this work, the good news is you can purchase a new BB Athearn type motor for about $15 or so and replace it, if the motor is roasted.
Not trying to pick at you either so don’t take this the wrong way. From the looks of your solidering, you need to do some practice work. Good luck and let us know what you find in the rehab work you have to do.