I removed the original light board. Then I hardwired the TCS M1. The M1 is mounted on the Front Transmission cover. TCS decoders lock using CVs 15 & 16. I haven't locked mine yet, I played with settings as I was working, so I could do one or the other at a time.
I also removed the Rear Transmission Cover. When you do that, there was enough room to fit the Soundtrax DSX where the Rr Trans cover was.
The speaker is actually below the Soundtrax decoder. Front edge of the speaker is very close to the motor. The sound goes down, pretty much through the rear truck. Since the DSX is over the speaker, it's part of the baffle. Then I just used Kapton tape to close off openings to seal and baffle the speaker. With the body off, you can barely hear it at all. When the body is on, due to the small size, the sound gets much better. It's still a little quiet, I will work on it more to try to improve it.
I don't think the cab mounting would be that bad. Sure, some amount of cutting and probably "opening" at least one or two windows to let the sound out. I figure you might get a 3/4" round with a small enclosure in the cab.
FWIW, change the 1.5 volt light bulbs right away and use LEDs. I tried resistors on the bulbs first, miscalculated the value and burned them out quickly. Doesn't take long to so the LEDs. They will mount right in the notch on the frame. Then use 1/16" clear acrylic rod in the headlight housing for light pipe.
I think it you go with the Soundtrax replacement board, it would make it pretty easy. The board would be a one for one swap, so no space is used up. Then instead of mounting the speaker IN the Rear trans cover, remove the trans cover. There will be plenty of room for the mini oval then.