There is a Karlson mod that allows you to make your own levels and publish them, ig its something new and i think it has great potential for speedrunning You can get it here if you want: https://rgc-exists.itch.io/karlson-level-editor

Mang already made one a little while back, you can find it in the modding channels of the discord server

That isn't even a Karlson level editor per se, it's based in Karlson, it uses Karlson assets, but the code is completely different; it's not a mod, it's a fangame. The movement in it feels whack. I made a level editor that's actually a Karlson mod: https://mang432.com.br/MangLevelLoader

Mang really hit em with the "it feels whack" 💀