Will the X51 airbox work on a 996. Will a standard 997 airbox fit in a 996,if so
im sure the x51 can be modded to work.Just need to cut second hole.Heres a link with a pic.
https://www.6speedonline.com/forums/...+x51+power+kit

 

In addition to cutting another hole into the deck lid, the air intake opening to the engine is a different size and will require an adapter, the wiring going to the MAF probably is too short and the oil filler cap/tube will also need to be changed to clip onto the X51 airbox.
Finally, I believe the 997 airbox is mounted differently into the car from the 996 , so you may have to do some adapting there as well .

It can be done, but probably is not worth the effort .