Here it is on the workbench (cluttered because I interrupted another project in mid-stream to do this).
Four screws hold the body to the chassis, located near the corners, like just about every other toy diesel loco. Here, I've removed the two on this end and you see only the holes.
After removing the tabs the body will not come off without some urging. These for hook-like tabs hold central body to chassis. If you can't pull them loose by just grasping the cab and pulling, you can reach them through slots in the chassi, underneath, and push them back from engaging a bit with a screwdriver tip, etc.
Before removing the body though, pay attention to the railings. On mine, all of the places where they touch the body were loose - they were a not-too-tight slip fit into small holes made for them. The railings are plastic, and flexible, and I just pulled them loose, then removed the body.
There are two boards, one mounted at each end, each directly over a truck. The larger appears to be the motor-control board and the smaller the sound board. This is the motor control board. Two screws remove and it loosens and you can get at and remove the screw to the truck underneath. Note the two clear-plastic insulating washer. If you remove this board (NOT necessary to wire the motors in series - I'm doing it just to have a look) BE SURE to put those insulated washer back on when reassembling.
Not much to see here, and confusing, four wires go into the truck, two red, two black.
Here we are with the board attached (and the two washer in place) and after a bit of diagnosis with the multi-meter. To the right are the wire connectors that bring power from the from the pickups and wheels of each truck. RATHER confusing on my loco, the wire colors are switched: on one truck, the red is the outside rail and the black the center, but on the other, the black is outside and red is center. Not important though, 'cause you don't need to fuss with them. The leftmost two clips go to the power leads to the motors.
I simply snipped the black lead from one and a red lead from the other and tied them together . . . the motors are now in series.
I reassembled the loco and only then noticed a potential problem. I;m certain this damage came from the factory. I pushed the roller and its pin back in and resolved to fix it later, but go ahead and test the loco now.
Here is a very short video of it running. Those who have one, unmodified, with the motors still in parallel, will appreciate how much slower it is running. this is the very first time I fired it up and I think with practice I could adjust it slower still, but this is a good speed. I ran it only this distance in deference to the very bad roller. I have a spare somewhere and will replace it later this afternoon.