I found it just by Googling "X-Men: The Last Stand script" and it was the first search result from the website Script Slug.
https://assets.scriptslug.com/live/p...stand-2006.pdf
But looking at it closer, I'm wondering if it isn't a fake. It's written by Dan Marcus and Kenneth E. Carper who I've never heard of and the bottom of the first page says DRAFT NO. 6 11/07/2006, which is after the original movie's release date. And it also says "Based on the screenplay by Simon Kinberg and Zak Penn," which seems to further indicate that this is just a fan script after the original movie came out. In which case, sorry for the false alarm and ignore my above posts about what was intended with Cyclops. Maybe James Marsden made the better choice after all sticking with Superman Returns.