Hi
it is a nice conversion but its a big job as I've put a 1jgte in my lexus is200, you need the mountings from the is300, you need a R154 box and get mountings for that gearbox and the ecu from a is300 or your GS300 its just splicing it into the is200, getting a is300 harness and ecu would be a better.
if you using the using your gs300 as a the one you taking the motor from then at least you will have the wiring, just use the transponder from the gs to work with the is200 once you put the 2jz in her cause youre key is encoded to the is200 ecu
you need the the oil sump from the is300 motor or get a 2jzge as the standard sump will catch on the crossmember. you also need to install a return fuel line
the proper shaft from a is300 or a modified one, the exhausts changed aswell also from the is300 or performance manifold
cool anything you need help me just message me