I'm not certain about the mechanical part, but there were some pretty serious frame changes that came with the Hexhead, so I doubt it could be done easily. What might be a bigger difficultly, however, would be the CANBUS electrical system of the Hexhead motor being very different from the electricals of the Oihead.
Also, looking at the transmissions, an Oilhead Tranny will definitely not come anywhere close to bolting up to a Hexhead tranny, so that means you would have to go with an entire Hexhead driveline - all the way back to the rear wheel. It looks like what you're talking about is not possible as an engine swap, but only as building a custom motorcycle, with all the complexities therein.
If you do a search about C engine improvements, you'll find that some people have put the airbox, cams, and throttle bodies from GS models on their bikes, and they really like the improved upper end HP. If you're looking for a performance boost, that seems like a far more practical approach. If you're looking to get a Hexhead, then buying a Hexhead model is probably the only sane approach.
Edit: here's some photos that show some of the frame difficulties (right click on the C and choose "view image" to see the whole thing, then click on the "+" to expand it to full magnification):
