It depends on the engine in question.
Diesels I've done and are pretty easy. Just take out the existing circuit boards, put in the new components, and wire accordingly. New lighting will be required.
Steamers are a bit more involved. Check if the engine has a Seuthe type or fan driven smoke unit. The latter is ready to use with GRJ's Super Chuffer circuit board.
You'll also need at least an 8 pin connection tether from engine to tender. 4 pins more than likely won't be enough to run between the two. Again new lighting will be required, though the coil coupler can be used if you're fine with its length. If the steam engine has a metal body and no plastic coal load (unlike the plastic body diesels), you'll need to insulate the shell from the frame with Kapton tape so the body serves as the TMCC antenna.
