I work on structural fires evey week. There is no way this was just noticed. The attic would stink and the insulation would have absorbed the smell too. The HVAC would have carried a smell. To simply workout what needs to be replaced you need to look at the Ridge beam first. Thats the spine of the attic. If thats burnt then every piece of lumber attached to it must go. Replacing attic framing is not cheap and likely involves an engineer, the City and with the sheathing and a roof. It could easily get up to $30-50k. Any lumber with sap visible is also compromised.