Rear tub repair

Whilst getting my 90 ready for the road again I have found the panel where the door striker plate is fitted to be heavily corroded at the bottom where it meets the sill bar and cracked around the striker itself.

Is this panel part of the rear wing if I buy a replacement or can it be bought on its own?

I've searched the tinterweb and asked several parts suppliers. But no one has replied

its part of the tub, where are you I have an old tub that has been used as a souce of ally that the panel could be cut from - pm me