Find someone that can weld is what I'd do.
Go in from the bottom and cut that chunk out, clean up the top really well. Then weld in a small plate of metal to give it it's strength back. Use some eastwood or similar rust converter to try to stop further rust and smeeer seam sealer all back over the repair.
Ideally you'd maybe drill the spot welds out of the strut plate on the bottom, then cut a larger hole and replace the metal. Re-weld the plate in place with new spot welds and coat everything to prevent further rust the best you can.
I cut out metal on my old car and welded in new plates around the strut plate, it worked for what I wanted to do.
