Plug-n-Play decoder, non-Plug-n-Play Loco?

[Squidbait]

Those are some scary-tall gear towers! They must come up to the bottom of the cab windows?

Not a lot of clearance over the top in the hood for a speaker there, I think. Therefore I think you're going to need to go with a deck in the cab just over the top of the gear tower, and put a speaker over/under it (depending on looks vs. clearance). What's the fuel tank look like? Any chance of making that your speaker enclosure?