If you actually want a proper functioning rear diffuser you are probably going to end up being charged more than the value of your car to produce it...
It will require huge amounts of computer design, then prototyping. It will also have to be tested in a wind tunnel to check that the diffuser is actually working properly and not reducing downforce due to incorrect angles/airflow.
You really shouldn't be announcing that you drive at 120mph on the road in the rain. 120mph is a dangerous speed in any conditions, but it would only take a few metres of surface water at that speed and you could kill yourself and others.
What you just said there should have been labelled "opinion:"
Everything Mikey said is perfectly valid, as it would be much easier and cheaper to modify existing parts than to create a whole new design at this stage in the cars lifetime. However that is his opinion, just like your opinion is that you should stop the IS200 looking old, by making it look like a Porsche (which hasn't really changed much in about 25 years)
There are many different bumpers, skirts, lips etc. available for the IS200, and I've hardly seen any of them in real life.