I've got a gen 4 3SGE from both a celica and MR2.
Currently running the celica engine with MR2 bits.
What are the differences?
1. Coolant neck.. Celica gets coolant filled up at the radiator, MR2 has a coolant filler in the engine bay. So the MR2 coolant neck on the engine block has 2 pipes instead of one
2. Crank pulley. Celica crank pulley has accomodation for running powersteering, with a half sized pulley wheel. MR2 one does not
3. Dipstick faces a different direction on each
4. Badges are the other way up on the manifold/rocker cover
5. Wiring loom is completely different, obviously
6. RHS engine mount was different
And that's about it.
It's definitely not impossible to use an MR2 engine in a celica or vice versa, but I wouldnt reccomend it, if you could just get a celica engine instead.
Although in saying this, I'm fairly certain that if your car came with a gen 3 3SGE for example, you could use the crank pulley/engine mount/coolant neck on the 3SGTE you're putting in. (assuming you're putting in a gen 3 3SGTE...)
The dipsticks are different lengths between turbo and NA IIRC.
I cant see why a GTT engine would be hugely hard to fit in a celica.