Why isn't the Kawasaki ZX6R on the list?

Hey guys, I'm building a F600 / FMod car from scratch. I've long been a fan of Kawasaki and wondering why the ZX6R isn't on the approved engine list in the US? It's on the UK and NASA lists.

I know there was a string of 636 cc engines, but I'm planning around a 2009 FI 599 cc from a wrecked bike.