I just installed a V5 Loksound decoder in an F7. The particular locomotive was bought used and someone had already done some milling for a rather large Digitrax decoder. The motor contacts were clipped off so that the frames didn't touch them and a channel was made up one side for a wire to run in. I'm not going to disassemble the engine to photograph that, but its pretty straight forward. It should be doable with a Dremel if you don't have a mill. On top I enlarged the existing pocket to fit both the speaker and decoder. The metal is pretty soft, a piecing saw didn't have any trouble. For the decoder pocket I used the Dremel since I didn't need a full width slot.
The tricky part was getting power from the frames to the decoder. I removed the light board and replicated it with a piece of thin pcb. I gapped this down the middle and soldered one wire either side. I think it was slightly loose, so I tinned the whole surface to give it a tiny bit more thickness. That was put back where the light had been, but now there was no light. So I took another thicker piece of pcb and made a little circuit on top of it, an led, a resistor, and two contacts for the wires. This was glued onto the frame contact board in such a way that it didn't short out. I added a capacitor in the cab, then it was a pretty simple speaker enclosure and we're mostly done. Bit of Kapton tape here and there, programming, sounds pretty good.
