Is the cab roof removable?
You can hardwire a TSU-750 to a NMRA 8-pin plug and plug that into the board in the tender (just remove that old Bachmann DCC decoder first), and probably fit the capacitor (you'll want to) in there too, but don't know if you'll have room for the speaker in the tender too.
If the cab roof is removable, and you don't have that locomotive at eye level or higher, then you may be able to get away with a small (the 1" 810112 or so oval) in the curve of the cab roof with a home made enclosure.