I sort of did what you are contemplating with a set of LifeLike plastic frame PA-1 units for a friend. I made one unit into a "dummy," removing the motor and gear towers, and stashed the decoder and speaker there, with wires running to the front PA unit to power it, run the headlight and route pickup power back to the second unit.
I used an ESU Loksound 5 micro; a Soberton 8x12mm speaker in a custom enclosure made of .030 styrene, and the Alco 12-cylinder 244 sound file from ESU (there is no surviving 16-cylinder 244, so the 12-cylinder will have to do). Sadly, I didn't take photos of all this, so I can't show you where I stashed all the components, but my recollection is that I put the decoder in the back, speaker in the middle, and keep-alive caps in the front. The units were coupled back to back, so wiring ran from the back of the decoder unit to back of the front unit. I found that running pickup wires from the front unit to the decoder unit was a necessity, because once you strip out the weights, power pickup on these things is marginal at best.
John C.