For the DEY-3 (S-1) locomotives, I'm using the Proto 2000 model. Mine are the pre-Walthers, my understanding is that Walthers made some modifications to the drive/chassis with a shorter motor, which would alter the process I used slightly.
The models themselves need very little in the way of detail alterations, with the biggest one being the cab. One of my shells (well, three of them, I have two spares) are at Chris' but Mike Redden is finalizing a new set of radiator louvres for the earlier version of the locomotive. So I'll cover all that in detail in the future.
After a couple of evenings of experimenting and testing, it took me an evening to do each one. I don't rush, and do a lot of test fitting and tweaking. I also ended up completely disassembling it to the frame so I could properly clean the trucks and to keep bits of metal from getting into the gear boxes while modifying the weight and frame.
For soldering, flux then tin each wire, and flux both wires before soldering them together. If everything is tinned and fluxed it goes very quickly and cleanly. I use a no-acid no-clean flux. Don't forget to thread a bit of the shrink tubing on first.
I wanted to use Scale Sound Systems' speakers in these (and future) locomotives. They appear to be 3D printed enclosures and have the speaker preinstalled. Most importantly, it is designed to fit a specific model by an audio engineer. His instructions specifically note that you should not modify the enclosure to make it fit. Of course, that means I did. I don't think these are problematic, but YMMV.
The instructions JT provides has you remove the weight altogether, and install the speaker using screws that used to hold the weight in place. There's a gap under the speaker on mine, and I suspect that's due to this being the older chassis. It's unclear in the pictures on the site whether a spacer was used under the front.
But I wanted to keep as much of the weight as I could as well, since these will have to haul the New Hartford Local up the helix. Out comes the Dremel.