My better half works at a certain BBQ chain with Galore in it (a little far away from where you are...!) and they have on hand / can order in replacement baffles if that is what has burnt through. The baffle sits on top of the firebox to (a) reinforce the top and (b) increase the surface area of metal to be heated and then have air blown over by the fan. You would need to know the make and model of your wood heater.
If it is the actual firebox, my understanding is that it is not possible to repair / is cheaper to get a new heater.
Cheers
Hafgan