Most of the Ringfield style motors are quite simple to convert to DCC, but read on as there are rare exceptions.
I once looked at converting a Hornby tender drive LMS locos and good job I tested it as when converting many of the same type of motor to DCC one assumes they are all the same and nice and simple so one may not think to test each individual motor... (As in ones mind one is thinking they will be the same) BUT good job I tested this one loco motor. It is one that the screw makes a hidden contact. Puzzled me at first as it is not visible and as the others do not make electrical screw contact one may not at first see what it is that is going on!
This was before Peters Spares came out with the nylon screw. I decided to sell the loco to a friend for his DC layout as I was not into LMS and was into BR blue diesels so it was a solution.
But yes. Make sure you test these Ringfiled type motors individually to check one can isolate each motor brush contact from the wheel contacts.
The only other loco that stumped me when I was converting my fleet to DCC was a Wrenn class 08 which today I don't want to convert it to DCC but I had considered it in the past. Was the rare locomotive that I looked at it and did not know where to start.
(I am told they can be done. I did not look any further and put it back together as it is!)
I also had a pair of early Bachmann class 03's which I did convert to DCC and they worked well, but I was not entirely happy with the way I did it because I wanted to use a proper screw and at the time I did not know a way of screwing a thread into a mazak split frame chassis. (I used the only other way I could think of and it was to wedge wires using tape inbetween the chassis and the body if I recall correctly. Worked fine but was not a satisfying "Proper" job).
Now I am going back to DC with 7mm narrow gauge, I may convert these two locos back and scale them up a bit.