Why the M7? Well, in my mind it's not an M7 at all, but an ex-Caledonian 0-4-4 Class 439 that worked as bankers at Beattock. Since there's no RTR version or kit available the M7 is the closest I can get without scratch-building (one day perhaps...).
The basic conversion approach is that described in Railway Modeller June 2006 by Roger Miller. Some of the pictures in the article are unclear (they were to me anyway) so I've included my own here.
The basic steps are:
Remove the body from the chassis, and clamp the chassis in a vice or similar (I used one of those crocodile clip stand things).
I used a Digitrax DZ125, bought for £17 from Digitrains.
I used a pointed soldering iron tip, for accuracy.
First step: unsolder the suppressor coils. There are two, and they come away quite easily by applying the soldering iron.
A removed suppressor coil.
Removing both coils it looks like this.
Then unsolder the capacitors (those little orange blobs). Here's the prepared chassis resting on the handle of my wire cutters.
You need to remove the yellow and white wires from the chip. No going back now...
Solder the wires, one at a time, ensuring the bond is strong (ie it stands a gentle tug - don't yank it!).
The chassis with all four wires soldered on.