Notepad Next - A cross-platform, reimplementation of Notepad++

At some point I was watching a sethbling video about programming mining turtles in a Minecraft mod called computer craft, which uses a reimplementation of lua with some mod specific functions. This inspired me to learn lua, then as I got bored with that I started moving through different languages until I settled on primarily kotlin and rust

So i recently inherited some legacy code.
Its actually not to bad. Just a few thousand locs which are mostly stretched across a handfull of functions lmao (800lines per function yay).

So the main thing i wana ask. Does someone here know of good techniques to gradually reimplement all of this.
Since im not gonna apply bandaids to this mess anymore than is needed.

Unfortunately this is a very important system and it only runs on production xD.

Idealy i would somehow be able to duplicate the tcp traffic to the reimplementation but that doesnt seem feasible.

Also what the individual modules classes and so on do wa snever documented and no one even knows how or why certain things even exist.

If anyone has any idea of what i can do. Apart from hoping to god i dont miss any weird quirky edge cases. Do let me know