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My guess is you would need the engine, tranny, DME, wiring harness, etc, etc, etc.. Cheaper to sell the C4 and buy the GT3 than to try and create a frankenstein car.
I seem to recall someone making the swap quite some time ago. They came to the same conclusion regarding the price. I think they ended up spending close to $60k to make it all come together.
I guess it all depends on what is more important to the OP. Is it the stronger motor of the GT3 or is it the all-wheel drive and transmission of the C4 tip?