Yep.. you would need OEM parts from the GTS actually, mostly the strut bar (would need to drill firewall), front sway bars, rear sway bars, front serviceable struts (rears are the same).
As for motor, you could go with either Bigport or smallport 16V 4AGEs or go to a 20V 4AGE or even a Supercharged 4AGZE
, you would need the full Engine, transmission, manifolds, ECU, harness, AFM (unless its a MAP sensored motor), and most likely upgrade your fuel pump (even if the sedan FP works, I'd upgrade anyways) and probably the drum brake to disc brake conversion on the rear.
This is something I actually have considered as I wanted to do the same from a GTS to a sedan, but I'm looking at other possibilities with my 89 GTS.
