Is it difficult to fit a DCC decoder in to a DCC ready engine?

In theory, it should be easy as the whole idea of being DCC Ready is to simplify the process.

The first time I did this, it took me less than five minutes however it will depend on where the socket for the DCC decoder has been placed. Sometimes they can be inside the body of the loco sitting on the chassis, some can be behind the smokebox door or even in the tender.

The service sheet from the engine will tell you where it is and how to access it. You may have to remove screws but overall it should be straightforward.

If you are unsure, your local model shop will be able to provide advice.