Stunned Silence is Very Reassuring

Understood by the lack of response that the answer is to abandon this endeavor and just get a DCC ready loco. 

Heard the word...

Dear Alan,

Trust your astuteness RE the response-received,
(Cue Clint Eastwood from "Magnum Force" in 3, 2, 1...)

Good move consulting the TCS Installation webpages, I would also reccomend bookmarking the "SpookShow N scale" website as a primary source of info. In this case,
http://www.spookshow.net/loco/bachgp40.html

I note that the 2019 GP40 release from B'mann was available "inc sound",
which is arguably the straghtest-line from "where you are" to "where you want to get-to"...

As far as "Decoder Buddy" motherboards are concerned, they are intended to replace the factory-circuitboard in locomotives (or add a motherboard to locos which pre-date "DCC Ready" form factors),
and make installation of 21-pin DCC decoders very simple and "clean"...

...the problem is that the 21-pin decoder form factor is over 15mm wide, which would be a stretch for a N scale boxcar or full-width F-unit, and is far too wide for anything resembling a "hood unit" or switcher...
(IIRC you'd be looking for a decoder and speaker which is < =9mm wide?)

Point being, current Decoder-Buddy offerings are designed for HO scale ,
(where scale-width GP/SD/Switcher hood units typically have more than 16mm internal width available,
and the 21-pin DCC decoder form-factor has gained considerable uptake in recent times...)

and are not-relevant for N scale installs at this time...
(although Nick is nothing if not "always thinking" about what-comes-next...   ).

Pls don't mishear me, I'm sure it would be possible to add DCC+Sound to such a loco,
if One was particularly skilled/naive and simultaneously suitably-motivated,
(I mean, ppl have installed Loksound + sugarcube speakers in MT N scale SW1500 switchers!?!?!?),

but if you're not-comfy to take it on at-the-moment, I really wouldn't reccomend it as a "first install" project...
(It's far from "plug n play", as I suspect the mass-response hinted to you...   ).

Happy Modelling,
Aim to Improve,
Prof Klyzlr

PS if you really want DCC+Sound, and it has to be in _this_ locomotive, you might also want to consider contacting a known reliable N-scale DCC+Sound installer to do the work?
(I am not directly-connected with any of the below, just "heard good things" about their respective services,
esp RE N scale sound installs.... Your Mileage May Vary....)

https://www.scalesoundsystems.com/installations
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http://www.dcctrain.com/shop/custom.aspx?recid=2
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http://www.tophobbytrains.com/installationservice.aspx

The response above looks very

The response above looks very well thought out and complete. I would add that the lack of response might not be an indication of the feasibility of your project, but a more general indication of the number of n scalers on the web site and to further reduce the number of possibles the number of folks with experience installing decoders in N scale. Not a knock on the N scale folks they are smaller in number compared to the HO folks and since their equipment is so much smaller I suspect there are fewer folks doing the things you are asking about.

There are lots of folks that do not tackle projects like that in HO scale and it is much more user friendly than N.

Best of luck with your project, and sorry I had nothing to add on your how to question.

Thank you

Thank you Prof_Klyzlr and Rob!