Well, that could be a bit unnerving...
Someone had posted a picture of a tear in their spring mount last week.
I suppose we now know what happens when the mount actually gives way.
Unless you are equipped to weld, try a welding shop. They are generally cheaper than body shops, and no points for aesthetics under there. It should come out stronger than new. This is really not that bad as structural repairs go. Easy access, and limited scope.
Certainly have the other side checked, and perhaps pre-emptively reinforced at the same time.
May as well inspect the balljoints and CA bushings as well, since the spring is conveniently already out of the way!
Best of luck.
1986 300E 5-Speed 240k mi.