Version 1.2.1 of my Galactic Marines mod which replaces the stock Republic 501st side in all maps by replacing the stock rep.lvl file.
To install, download the galactic marines 1.2.1.rar file and extract the file inside labelled rep.lvl to Star Wars Battlefront II\GameData\data\_lvl_pc\side.
BACKUP WARNING - This will override the existing rep.lvl file, so be sure to copy that to your desktop first so you can revert changes!
The default location for you side folder is: C:\Program Files (x86)\LucasArts\Star Wars Battlefront II\GameData\data\_lvl_pc\side. Or if running the Steam version of the game: C:\Program Files (x86)\Steam\SteamApps\common\Star Wars Battlefront II\GameData\data\_lvl_pc\side.