How difficult is it to replace the fuel system cut off solenoid?

are you talking about a diesel? if so take it to a tech as putting it in upsidedown is easy to do and will ruin your engine n0rt The fuel cut off solenoid is a simple bolt on, however long slender hands and fingers are a great asset. If installed incorrectly the engine can 'run away' meaning rev uncontrollably until destruction. The stop lever will not work in this situation. I suggest you have another person start the car while you prepare to block the turbo inlet with a damp rag over a small piece of 2x4. Should the engine attempt to 'run away' you can block airflow and suffocate it to stop.