However, is not an official SNK/Playmore release. It was distributed through early ROM-sharing sites (late ’90s/early 2000s) as a patched version of the original. If you’re looking for it now:
The King of Fighters series has a rich history, and I'm excited to dive into a story based on your prompt. kof 99 plus rom exclusive
One of the most significant features of specific "Plus" hacks is the ability to toggle between different game styles. However, is not an official SNK/Playmore release