MAX+PLUS II Software download for MAX9000 Series

That part series went EOL over eight years ago, back in 2016. But you probably know that.

What happened to the original design environment you used back in the day?

You had to have a Windows XP (probably) or similar PC with all the software installed and running.

Try this: https://archive.org/search?query=subject%3A%22maxplusII%22&sort=title

and see if you can find something usable.

Maybe Intel/Altera will let you sign an NDA and give you a private download (if then can, who knows?).