If you get a replacement second-hand chassis, the identity of your vehicle changes to the numbers on that chassis. In which case it is most important that you have the ADR Compliance plate for the replacement chassis.
I gather by your comment, you are happy to repair your current chassis so the issue of compliance plate is irrelevant.
Repairing the outrigger. Britpart make numerous chassis repair parts you can use, however looking at the firewall support outrigger, you may find that you can repair it by merely removing the rear element and welding back a replacement. You may even be able to make a tank support and outrigger repair in a single piece of folded steel.
Craig, where do you live? I am currently wrecking a rollover ex-Army SIII and we may be able to donor chassis parts off it.

