Applying multiplayer settings to single player essentially just relies on a glitch where if a game being hosted on GameSpy is cancelled before it's listed, the gameplay options hosted will get applied to the next game you start, regardless of whether that's a single or multiplayer session. As GameSpy is down, we'll need to patch the executable to use the OpenSpy servers for this to work. I've also only tested this on patch 1.2, but you'll want that anyway as it's an official patch the Steam release did not include for whatever reason. So without further ado, here are the links to the other guides to go through before this one.

Patch installation.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=326627023
8+ thread CPU fix.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1869921614
Online multiplayer patch.
https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=1654378740

Firstly, we'll need to make a preset for single player with the settings you want to apply. Not everything works so you'll just have to experiment with it and figure out what you can do.

Navigate to the multiplayer menu, create a match and hit edit to look at your presets. Create a new one for use in single player mode.


Now change the options you want and hit save. I'm going to increase movement speed and jump height.


Select your preset and hit next here. Map and mode don't matter.

Make sure network mode is set to Internet, then hit next.


Now click cancel when you see this message pop up. You have a short window of opportunity to do this.


That's it! Load up a single player game and see if the mutators were successfully applied. Then go ahead and shift on Krone's goons. The settings will stick until you back out to the main menu or quit the game.

I get "Unable to start the host"

Most likely your game is still using GameSpy instead of OpenSpy

The game starts immediately

Double check you have your network set to internet rather than LAN.