Can a termite inspection report declare a house irreparable?
Bring in an engineer. The termite inspector's job is to tell you that there is damage and where. The engineer's job is to tell you how that affects the building and what can be done about it. (A home inspector likewise is generally tasked with pointing out what's an issue rather than telling you how big an issue it is, though you can sometimes read between the lines, especially if you walked through the inspection with him.)

Damage that affects structural members of a house may not be repairable. Folks in my neighborhood had to demolish an old farmhouse they'd bought because it had a combination of problems (not termite-related), and nobody was willing to try to fix either one for fear of causing the other to fail catastrophically.

(The termite inspector may give you a general sense of whether the damage is cosmetic, minor, or serious... which will at least tell you whether you do need the engineer's opinion.)