G'day 69gs
I hope that you changed the oil BEFORE you stirred up that crud in the sump, and splashed all that acidic oil around
that firewall looks OK to me, just a little bit of rust on the Left side, mine was worse than that on the right side, I just handmade plates then welded them in out of 2mm steel,and salvaged the pillar sections from another old rolled firewall
but those sections are available now as rust repair section, both here and from O/seas ( better quality) and then you can replace the dead sections, the inside sections can be excised and trimmed with a die grinder and repaired
cheers