Be more specific dude! I thought you meant headlights, doors, etc.
the IS300 comes with the 2JZGE motor which is RWD and was Also used in the Supra non-turbo, it has been fitted to the AE86 before with custom work. Actually I've seen 2JZGTE AE86s before, which is from the Supra Turbo. The Altezza in Japan came with the BEAMS 3SGE which has ALSO been fitted to AE86s, AE92s and AE101s before with some custom work.
Exhaust could probably be custom fitted too, I'm actually looking for an IS300 OEM muffler to weld to my car.
So yeah, with custom work you can fit parts if you want.