Len
That sound decoder is an HO one, it has more than enough grunt to drive a coreless motor especially the 6 watt Maxon that ABC supply and a reasonable size speaker (but not a massive one) I would recommend one of the cube speakers you can get from Zimo, these are very good and work well with that decoder. You may also need a Plux 22 converter board (ADAPLU50 or 15) to plug the decoder into and then connect the wires to that board they are bit bigger about 45mm long but the same width, this also boosts the current limit to 1.8 amps.
If it isn't the Plux 22 version of the decoder you will need a 21 pin MTC adapter board.
There should be plenty of room in the tender but plan it out before you start building, trying to hack the innards out of a built tender isn't fun.
Richard