The Santa Fe that I think you are refering to was not a failed railroad. It was one of the largest Class 1 railroads, and it merged with the Burlington Northern to form BNSF in the 90’s. The Santa Fe (which is probably the Santa Fe Southern) that you found in New Mexico is not the same.
To answer your conversion question…I’m not really sure you can get where you want to go with your starting point. The Santa Fe loco you pictured has two cooling fans and exhaust stacks (non-turboed) on the dynamic brake hatch. The NS that is pictured has three fans and a turbo charged exhaust hatch. The side grills are different. There are other differences…a lot, as a matter of fact. I think it would be much easier to start out with a model that is much closer to what you want, or just repaint what you have and call it done.
Now having said that, back in the 80’s, MR ran a bunch of articles by Tom Busack. He was great at cutting apart shells to come up with something completely different. If you have access to MR’s archives, I would start there. Just do a search on his name and you’ll find some info that will probably be useful.
I agree with Chuck. The Santa Fe loco you have pictured is of a AT&F (Atchison, Topeca and Santa Fe) GP38 or a 38-2, which is now merged with the BN to form the BNSF.
The NS loco is a model of a GP40 or 40-2. If you Google “BNSF GP38”, you’ll find lots of pics that you can use to up-date your Santa Fe shell as is, with no changing except paint and detailing.
That would be the best way simply because those locomotives are from two different manufacturers or a upgraded run… The 3500 has truck mounted couplers the NS has body mounted couplers.
First you’ll need to remove the shell, and then remove any “glass”, and handrails, and the sheet that came with the loco or train set, should show an “exploded” view, to help you in dissassembling the loco.
There have been a few threads on here on paint stripping, but I have had good luck using 91% ISO alcohol. Let it soak, and then scrub off with a toothbrush. Other guys have had luck with brake fluid, and even Oven-Off in the yellow can.
Wash the shell off in soap and water to remove any residue, after the stripping, make sure it’s completely dry, then paint.
You’ll have to search around for a NS GP38 or 38-2 decal set. I just did a quick look through eBay, and didn’t see any.
You can also add any aditional detail parts you wish to, such as the plow, hoses, etc.
I’ve done a few “rebuilds”, and it’s a satisfying feeling once completed, and the loco makes it’s first run in the new paint scheme.