Did one hour tonight.
Removed the rotted 20mm plywood out from behind the dash and took measurements to make a replacement piece.
Am going to have to cut most of the damaged dash fiberglass out and re-lay. Contemplating whether to change the shape of the dash and extend it lower to make a larger face area.
Showing the rotted timber behind the dash:
A pile of wet and rotten timber. I kept the ends to use in developing the template for the new dash panel:
Under the dash with timber removed: