I would never normally suggest anything like this, but if the stone is (a) unprotected (b) in danger of destruction and (c) small enough, what about the posssibility of removing it until it can be safeguarded?
I can hardly believe I'm typing that, so by all means shoot me down in flames over this suggestion. But those tyre-tracks look unpleasantly close.
IF the site can be returned to anything like a presentable state, then at least there will be SOMETHING to return there, if only as a badge of shame to future generations.
What would the original builders want you to do? Because if no one else will protect what they created, then you have effectively taken their place for the moment.