i have no experience with that loco, nor with the 3s-21, but given those provisos, could you try this:
1) save the 3s-21 for another project and purchase a 3S.
2) remove the printed circuit board and all the white connectors, identifying all the wires (track current, motor leads, lights, etc)
3) hard wire the 3S to all the wires that were previously connected to the now-removed circuit board
4) use the smallest speakers you can find (i've used iphone 4 speakers which are pretty thin)
if none of this is applicable to your model than feel free to ignore the advice!
good luck
Ken
ps--if you are still short on space in wedging in a 3S, you can remove the plastic housing (voiding the warranty) and wrap the module with Kapton tape which will provide electrical insulation.
