I replaced the steering and control cables on my old 204C many years ago. I don't recall needing to unbolt the engine. It wasn't too hard of a job. The transom mounting bracket should also be replaced. As I recall, the length was 18' for the steering cable and 22' for the control cable. But my near perfect memory isn't what it used to be, so either try to read the length on your old cable or give Grady a call.
This might be a good time to upgrade the steering to a modern no feedback system or hydraulic.