Suspect broken defrost timer

I've had problems with this frig. almost since I got it. The ice maker has been broken for years. I think the problem may be a valve since the entire ice maker was replaced but it is still not working. The current problem...frig and freezer not staying cool. I had a service call and the technician said the Defrost timer replacement part was not available and the availability was not known. He recommended that I get a new frig. I'm not happy with that answer so I'm looking to see if I can do the repair myself.