Thanks to a search by Matt Forsyth I was able to purchase a "Seattle built" NWSL 654-6 23:1 ratio 5mm axel gearbox for my Sunset J1. I just completed installation and thought some may be interested in learning how I went about the project.
The first photo shows the OEM gearbox. Note that it used a 4 spur gear tower to drop the horizontal drive train down to the driver axel gearbox. The second photo shows the badly worn brass axel gear after it was removed from the axel.
I don't have a quartering jig so before I pulled the un-insulated driver off the axel I scribed a line across the hub of the driver and the end of the axel to indicate their in-quarter relationship. I used an assortment of common shop tools, and a piece of bar stock (slotted to go over the 5mm axel behind the driver) to press the uninsulated driver off the axel. A gear puller was use to pull off the worn out axel gear. The new NWSL gearbox bearings, drive gear, and right side frame bearing were then pressed on the Sunset axel. I found that the Sunset axel was slightly knurled at it's center (for the drive gear) and on its ends (for the driver faces) so some gentle force was necessary to press the parts into position. The most critical task was pressing the driver face back on the axel in an orientation that aligned the scribed line segments. The press was done using an inexpensive Harbor Freight bench press. Washers were temporarily stuck to the centers of both drivers (with grease) so that the press was parallel on both driver faces (no contact with the tires or counterweights). I made the press in small steps checking progress with a NMRA wheel gauge as I went. Before mounting the gearbox on the axel I temporarily reinstalled the driver (and main rods) in the mechanism to make sure there was no bind or wobble. Good fortune - no problems 
. The NWSL gearbox was slightly larger than the Sunset one and required enlarging the opening in the one piece driver bearing retainer plate (I used a file). A NWSL 495-6 universal joint kit was used to connect Pittman's 5/16" motor shaft to the 1/8" NWSL gearbox shaft. Per NWSL instructions "D" shaped flats were filed on the ends of the shafts enabling the sets' horned balls and slotted cups to be pressed on with no possibility of slippage. The new drive shaft setup slopes downward so I had to fabricated a short front motor mount. I was able to reuse the rear half of the original Sunset motor mount by bending it to the proper angle. To keep the gearbox from rocking for and aft (from drive train torque) I screwed a pair of brass flat stock restraining bars to the upper face of the frame. Overall it was an interesting project that took approximately 8 hours. The best part is that the model now runs smoother and quieter than when it was new.
Ed
PS - The gearbox I used was an older one made in Seattle. Like Jay I discovered that the gearbox halves were not drilled and tapped for the cover plate screws. Rather than waiting for a replacement I just drilled and tapped the gearbox halves using the holes in the cover plate as a guide.
Ed
