Hi....welcome to the Club
I would imagine there was something seriously amiss in the first place for it to catch fire.
If it is under the dash then it would involve a complete wiring loom, electronics, SRS and many other control systems....all of which will need programming to the vehicle (or whats left of it).
In my honest opinion I don't believe it would be worth doing, financially and otherwise.
If it has been paid out for by an insurance company then I would dispose of it via eBay or similar and look for another vehicle