Fleishmann Class 13 4-4-0 - anyone fitted sound?

[edam_cheese]

A friend of mine asked me to take a look at his Fleishmann 4113 DRG Class 13 4-4-0 steam tender engine (in HO scale) with a view to fitting DCC sound. I have converted a number of UK diesels and US steam to sound but this looked like an altogether trickier proposition. I'm sure someone must have done it before and I am looking for any advice. I think it should be possible to fit a loksound micro or Zimo MX648F sub mini in place of the regular decoder with a small speaker in the smokebox beneath the chimney. I noted that there was a metal back plate to the factory fitted decoder, does anyone know the purpose of this and if it needs to be retained? Any advice will be gratefully received, a Google search showed up nothing of any real help.