I looked at the 168 this spring while I was in Colorado. Considering that it has been 16 years since it was restored it looks pretty good. The tender tank was badly rusted when I did the work in the early 80's. I put in flush patches in all the bad areas at that time but it looks like some new areas have developed. They should be repaired by cutting out the bad areas and flush welding in new steel. The display was lighted at night (four lights in brick enclosures to the side and slightly below the engine) the headlight, marker lights, and the cab lights also come on at night (controlled by a photo cell).
The engine should be repainted every 5-10 years and any broken windows or other vandalism should be repaired immediately as it occurs. The best protection is being well lighted, well maintained, and loved by the citizens who frequent the park.
If a roof were put over the engine it would look more out of context and it would be even harder to get good pictures of the engine.