Canadian National had tenders with tall oil bunkers that would be perfect to fit a decoder into. Just scratchbuild the bunker out of styrene. I remember the conversion in the video, if you search around you can find how it was done. The speaker is in the cab, and it doesn't have a baffle which is why it sounds a little weak. I bet you can cram in one of the newer "sugar cube" type speakers in there. You can remove a signifigant portion of the metal boiler that extends into the cab, and make a custom enclosure for the speaker that is curved to match the cab roof. It can be done, but a lot of work.