THE KING OF FIGHTERS 2002 UNLIMITED MATCH has an alternate mode locked by default called "NEO GEO MODE", where you can play the original arcade version of the game.

There were several mods that enabled this mode in the past, but it seems Code Mystics patched them out in the newer versions of the game, probably because they don't want this mode to be played because of the K9999 controversial and removing from the series, since the KOF98UM still has a NEO GEO MODE unlocked by default.

What I'm bringing here is just the latest old version of the game where these mods still worked. The mod applied on this version I'm bringing here came from this guide. I'm not the author of the mod and I choose this mod specifically because it seems it doesn't only unlock the NEO GEO MODE in the game, but it also fixes a lot of bugs and errors that comes with it.

1. Go to the game folder (Steam\steamapps\common\The King of Fighters 2002 Unlimited Match) and delete ALL files inside it.

2. Download[pastebin.com]and extract the .rar file inside the game folder. It contains all the files of an old version* of the game with the mod already applied.

3. Done. Now you can play the game normally with the NEO GEO MODE enabled. Have fun and don't forget to Like and Favorite this guide if it was somehow helpful to you. :)

*Notice this is the lasted version of the game where the mod works. Further versions won't open or won't work at all.

If you want to reverse the process, just:
It should bring the latest version of the game back. If it doesn't work, reinstall the game.