I'm wanting to pick up speedrunning this mod as a hobby, but I'm not sure how to get SourceSplit working with it, and LiveSplit doesn't show up over the game, even in windowed mode. Does anyone know how to fix these issues?

Try open the livesplit with admin permission

and make sure you are using the right version of sourcesplit (check the rule, there is a link of sourcesplit)

Running as administrator worked for getting LiveSplit to show up over the game, but the timer still isn't going automatically.

You have to press a start key to start the timer, but if you followed the guide for the timer that is mentioned in the rules, it should do everything automatically after that (in-game time & splitting).

You also need to manually end the timer with the split key though, it won't end automatically either.

Alright, thank you.