once you get the motor isolated from the frame, you can fit the decoder at the rear of the chassis between the chassis casting and the firebox/boiler backhead on the body. There is enough space ther for a fairly big decoder up to 3-4mm gap thick, so you shouldn't need too small a decoder.
My usual method of isolating the motor from the frames is to disassemble the whole thing, then drill the frame out about 3mm where the motor brushes and the springs are located.
My usual method of isolating the motor from the frames is to disassemble the whole thing, then drill the frame out about 3mm where the motor brushes and the springs are located.
