Can't remember exactly how he did it, but my mate didn't have any real trouble welding the seat mount area back in... I fear mine is going in the footwell area too, just not as bad as yours! ....I hope!!!

You must surely be able to get a repair panel to do the footwell, even if you have to pay someone to weld it in (I can't weld for shit, so that's what I'd have to do!!) it would be easier than re shelling it, unless the rest of its rotten like that underneath!?
It's not GOT to be mega neat, so maybe you could hire a mig welder an have a bash yourself!?