Hello Simon,
I did it very simply - for the motor bogie, by pulling the original wheels off the stub axles and pushing Ultrascale ones on in their place. A lot needs filing off the inside of the frames but otherwise that is all there is to it. It could fairly be described as abusive treatment of the Ultrascale wheels but it works and is dead easy. Of course only Ultrascale wheels will do as they alone have extension bosses on the inside. I have done a CEP, an EPB and an MLV - all share the same drive mechanism. A big advantage of the converted motor bogie compared with the original is that all that dreadful sideplay gets taken out - no washering needed thanks to the bosses on the backs of the wheels.
For the trailers, the society stores do a replacement plastic bolster to space out the frames - the electrical connections in the side frames need to be split to avoid the axles shorting. Again I used Ultrascale wheels.
You can see that I shorted one side on each wheelset so that each wheelset picks up off one rail. You, of course, might have alternative solutions in that department...
Herewith a couple of quick pics. this is the MLV which must have taken all of an hour to do.
Cheers,