Having driven in my mates MX6 1997, I have to say that all in all its not a bad car, I mean it goes(abit nah its actuallly not too bad) but what I love is when he's thrashing it around and taking corners much faster than he should the front seats fully hug you and you dont move around at all(I know it sounds all soppy but thats how it is!!!), they are the most comfortable and supportive seats I've sat in! Is a seat transfer relatively easy, would the Mazda Seats fit? and whats a pair worth(roughly...)? Cheers.
You'll need an engineers certificate to be legal. The cabin space in the Mazda is a fair bit bigger than the Celica's, so the seats might be to big for them. You will have to manufacture some custom mounts as well.
Or you could get the celica seats reaphulstered with extra support built in. It won't be the same as flashy new seats but not such a bad compromise.