This should really say restorations in progress as once the work is complete it is elevated to one of the other sections of the site displaying the finished article and the steps needed to get it there.
When to restore and when to leave well alone. Suppose it depends on the individual but some engines are clearly too-far-gone to be any use otherwise.
One argument says that any self respecting engineer would do whatever is necessary to keep his engine both in working order and looking good. If there are worn or rusty parts they would be replaced restored and painted.
Another school of thought is that the engine's state is a reflection of it's past and should broadly remain that way. For example one would not rebuild the acropolis just because it is in a ruinous state. Fortunately none of the engines I have could be considered 'National Treasures' and I generally make up my own mind depending on the circumstances at the time.
An old nut-and-bolt Mamod steam roller undergoing the treatment.
Old Wilesco D5, looks like a bit of a challenge
This is the story of the repair of my Bowman M135 broken steam pipe.