How do you know when it is ok to restore a vintage item?

Restoration is a topic in the world of antiques and collectibles that is discussed quite a bit.  There is one question that comes up in the discussion—when is it ok to restore an item and when do you leave it alone?

There are a couple of things to keep in mind when it comes to doing a restoration.  The first is how complete the item is before restoration.  How much time will be spent looking for parts to help complete the restoration?  There may come a time that you may have to make a part (or parts), and this could drive up the cost of the restoration quite a bit.

The second thing to remember is the cost of the restoration.  If the cost is more than the value of the item, then you need to leave it be.  When you are figuring the cost of the restoration, you can also figure out how much it would cost to simply repair the item.

The third thing to remember when you restore an item is pretty simple—what is the value before and after the restoration and how much value will be added when the restoration is done?  If the value does not go up that much, then you might want to make sure the item is simply functional rather than completely restored.

What items do you know of that benefited from a restoration?