On rewiring these switchers, I have rewired 10 or so and have been happy with the results. From my experience I make the following recommendations. Do not use wire that is too large. There is not the space in this loco and the flexibility of the smaller wire is needed. I used 22 gauge solid push back cloth insulated wire. This is the same wire Lionel used when they built the loco. It would be best to keep the same colors as Lionel used, the two wires from the e-unit to the brush holders is yellow and the wire from the e-unit to the field connection is green. All the other wires are black. The wire from the front coupler to the brush plate should be cloth insulated super flex. The only color available is black, which is correct.
Where the wires go from the area of the e-unit to the brush plate is difficult routing as there is little clearance under the front cab wall and over the front motor plate. Three wires need to go to the right and three to the left. The power wire, which goes from the headlight, then to the two collector roller assembly, and on to the right hand plug on the brush plate, I make up as a single harness. Where the wire attaches to the two solder lugs I make a 180 degree turn and put this through the lug. Then fold the two wires over and solder them to the lug. Do not forget to slide the short piece of fabric loom over the two parallel wires before attaching the lug. This will slid over the lug after it is attached to the collector assembly.
The motor needs to be fully serviced, reinstalled and tested prior to connecting the wires to it. Using the brush plate as a terminal board was Lionel’s big mistake on this loco. It makes servicing the motor impossible without disconnecting many wires first. As I indicated above, remove the e-unit drum prior to soldering the wires to the e-unit to avoid heat damage. On the fiber contact plate, the lower left contact should be connected to a wire coming from the e-unit coil, then it goes down to a rivet on the e-unit frame. This wire provides the frame connection for both the motor and the e-unit. The connection up and to the right is where the green wire from the field is connected. The upper left and lower right terminals are where the two yellow wires from the brush holders go. The other connection from the motor field goes to the right hand plug on the brush plate, which is center rail power.
On using parts from the 18000, the 1989 remake, some times Lionel used the original part number on parts that are interchangeable with the original prewar switchers. Sometimes they used the original part numbers on parts that are not interchangeable. And sometimes they used new numbers on parts that are identical to the original parts. But the strangest one is they used the 701 numbers on the wheels, which are nearly identical to the original 227 wheels, not the original 701 wheels. I think they kept original numbers on all the gearing, but they changed the pressure angle to 20 degrees from the original 14.5 pressure angle. Gears with the two different pressure angles might mix depending on how worn the gear teeth are and how much slop there is in the center distance.
As for changing to post war couplers, on the tender this is usually done by putting on post war trucks. They have smaller wheels so the tender sits lower. If you can live with the way this looks, a “S” bend will have to be put into the drawbar to get to connect to the loco. To raise the tender on the post war trucks, there is a filler part that is used on the postwar crane trucks to raise the crane. It looks like a thick washer with a grooved pin on top for the horseshoe washer. On the bottom there is a pin that is intended to be clinched into the postwar truck frame. For the front of the locomotive, go up about 5 pages in this thread and there is information on how to make the front coupler out of currently available parts. The one part that might be a problem is the bracket that screws to the frame. I believe this bracket was used on other locos and can be found. I would have to do additional research.