I had a Hornby loco in the eighties and it was tender drive, guess yours was prior to them adopting this configuration.
By isolating the motor I mean sometimes it is wired through the chassis. If it is you need to isolate it otherwise the decoder will be spoiled, so you need to check this. Just look at the wiring from the motor, trace it back and see if at some point a wire is connected to the chassis. It may not so wired in which case your job will be easier. As I said post some pics of the disassembled loco and drive that would help you proceed.
Here's another video of a tank of a similar vintage to yours which may help you: https://m.youtube.com/watch?v=Ad9X0VRaNok
I couldn't find any of the engine you mention but the principles are the same.
I did find this from a company that does DCC conversions which looks like it could apply to your loco:
"This is an interesting conversion. The loco is an old Hornby Scotsman with standard 3 pole motor built into the live chassis. The motor terminal connected to the chassis is simply isolated by cutting the insulation collar to the insulated brush in half and using it to isolate both brushes".
So it looks like the motor needs isolating.