John, if you're looking for something to "patch" the old firewall, I've probably got a couple of candidates, but I'll be out of town for 4-6 weeks.
If you're looking to replace the whole firewall (I recommend it with any major firewall damage), you should definately go with a new Cessna stainless steel replacement firewall (the originals were all galvanized as Charles suggested). Except for a few alignment holes the rivet holes were never pre-drilled and were all hand drilled during installation, so no two used firewalls are alike and it will be impossible to produce a quality installation (rivet holes a half hole or more off, etc.). The new firewalls have only the pre-drilled rivet/alignment holes that match the old. One size fits all from C-140's through C-150's and C-152's. You will need to cut the correct heater holes, etc., using the old firewall as a template.
I have done this, and it's time consuming, but not that hard with the right tools.
