One thing to look at...

IF you can pretty much shove everything back to it's original position AND add some bracing, then you may not need to cut out the existing firewall.

A good epoxy joint can be far stronger than wood... so if the pieces of the broken jigsaw will fit back together you may avoid having to create a new firewall...

Not that I disagree with RCKen, but I've often done this with broken landing gear areas as well and the result is usually far stronger than the original if done carefully.

If the pieces do "fit" so to speak w/o significant missing parts or areas where you may not be able to get glue into you can....

- Harden existing wood items with CA or Epoxy & let dry, but be carefull to maintain the "fit".
- Epoxy everything with 30 min epoxy
- Make two mixtures one at full strength to apply to the broken areas
- The other mixed with Alcohol to permit it to seep into slightly cracked or exposed wood.
- Hold everything together with clamps for 24 hours until fully dry. Make sure that the original shape is maintained.
- If it looks and feels solid, I use angled hardwood "braces" along the joints as well, these get epoxied in after roughing the area up a bit.
- If this still seems unsound you can still cut the entire area out and install a whole new firewall.

Without pictures or viewing the damage though, it's hard too tell which way to go.