What's the name of the game engine? If it's something that's been used widely¹, a search on that name might turn something up. Otherwise, you'll probably have to go from the game's documentation (or its source code) to identify the codes and their meanings, then write something to convert them to HTML.

Alternately, if users are running it entirely through a web interface, you could tell them it's time to abandon the legacy formatting codes and switch to BBCode (or raw HTML, if you trust them enough) to format their names.

¹ ...this decade. The style of the codes gives me the impression that this may be a converted BBS door game and I have a feeling there aren't too many of those still running around these days.