To be able to paint the body and the chassis separately, I removed the chassis from the body. While the chassis was apart, I decided this would be a good time to install the flywheels onto the motor shaft. I had bought two NWSL 403-6 flywheels for this motor.
The motor is attached to the chassis with two screws. It then easily uncouples from the universal joint. The joint's part can be slid off of the shaft easily. The flywheels fit the motor shaft, but they require vice-type pressure to install. I don't have a bench vise, so I used a large woodworking cabinet clamp. I lined up the flywheels with the motor shafts by raising them up with small sections of styrene (not shown in the photo below). I could then apply the clamp's pressure to squeeze both flywheels onto the shafts. I tighten it up, then backed off to verify they were going on straight. I then applied pressure again and backed off again. I did this several times until I saw that the flywheels were going on the shafts correctly. It is absolutely critical that the whole motor/flywheel assembly is perfectly perpendicular to the vice or clamp you are using, otherwise you can bend the motor shaft! This is not a quick-before-dinner type of task; it requires absolute concentration.
Next, I applied pressure to have the flywheels go onto the shaft such that their ends lined up with the end of the shafts. The flywheels need to be pressed further up the shafts because the universal joints need to go back onto the shaft. What I did to accomplish that was to use some left-over wood and drill a hole in each piece to allow the shaft to go through the hole of the wood while still being able to provide consistent pressure from the clamps.
One flywheel slid further up the shaft but the other one didn't. I then use a metal weight instead of a piece of wood on the side that had gone through enough, which forced the other one to go up the shaft too (shown below).
I then replaced the universal joint and installed the motor back on the chassis and tested it. In all fairness, the North Yard chassis runs great without the flywheels. But I figured that it is relatively easy to install that feature now, rather than to have to worry about trying to retrofit that later.