I would suggest dropping in an ST205 Celica GT4 3SGTE as they are internally stronger than the earlier ST185 3SGTE and also produce more power stock than the ST185
but definately do the whole engine conversion... dont bolt a turbo to the 3SGE
Have a chat to the guys aty BD4's or Advan perfomance about the work involved. I know BD4's recently completed an MR2 where they started by dropping an ST205 3SGTE into it and going from there...