Depends on how bad it is.
Regular fire clay ( mortar) is really only meant to be 3mm thick max.
You may get away with parting it up. And you may not.
Finding a good bond to old firebrick is gonna be tough.
There are castables available for this purpose. But the cost of them might outweigh the price of just putting in new fire brick.
Not sure if you still have my email Bruce. But I am re-bricking my arch this summer and could give you a hand or do it for you.
I’m sure I’ll have lots left over from mine too. I’ve been saving them for years from various jobs.
L
600 taps on vacuum
Lapierre mechanical Releaser
CDL electric releaser
2.5 x 10 CDL Venturi ( new for the 2024 season )
Home made modulating auto draw off
Homemade RO 2 x 4" membranes
CDL 16 x 16 bottler
Wesfab 7" filter press
Delaval 73 vacuum pumps
12 hives of bees