Of course, you can take a look here:
https://github.com/koreader/koreader
You can compare version you have with current master. On the other hand, this isn't necessary easy thing to do, which change did what you don't want done.
But, you need to know that when you start reading book settings are kept in directory with book's name. So, you can compare and try to find which setting is that that turns hyphenation off.