In my experience you need to replace the actuators on the doors that are not working. I have bought secondhand ones from the scrap yards. They are easy yet not easy to replace as you need to unclip them and it is very awkward to get to the clip. Of course, you have to remove the door card and the dust cover and then disconnect the actuator plug. I used a screw driver to unclip the old actuator and remove it and clip the working actuator back in and put the electrical plug back. Trying to fix a non working actuator is a waste of time in my opinion. The door actuators are all the same left and right, so does not matter from which door you get them, they will fit on any door. I would spray some Q20 or WD40 into the lock mechanism while you have the door card off and grease the window mechanism as well.
If you are going to use the old engine and cylinder head, make sure the hole for the plastic elbow at the back of the head is not too badly corroded as the new pipe with the O-ring will leak. Let the engineering works weld up the hole and open it up again so that you have a nice smooth opening for the O-ring to seal on. I never noticed the corrosion on mine and had to get some special gasket former to get it to seal watertight.
