Hiya, folks! I just thought I'd make a post to help people who have been having trouble downloading/using OpenGL for the Mac version, since there are quite a few issues with it right now, but they're all fixable!
1.
Downloading
The link in the main post to download the Mac version is still broken, but you can still download it by going to the Releases page of the GitHub project:
GitHub Link
I can confirm it works on Catalina, too!
2.
Additional Characters
Some things don't quite work correctly on the Mac build, in particular the built in Additional Characters.kart pack. The pack is located inside the Application Package, but the game doesn't look there for mods for whatever reason. Instead, it looks in the newly-created "srb2kart" directory in your Home directory (The folder that has your username as its name).
So to move it to where it should be, go to your Applications folder, right click on the game's icon and select "Show Package Contents". Then navigate to /Contents/Resources and you will find the "bonuschars.kart" file! Simply copy & paste this to the "srb2kart" folder in your Home directory and you're set!
3.
OpenGL
Since macOS typically only uses one executable per application package, there isn't an obvious way to start the game with OpenGL to use 3D models. Thankfully, forum member Lach found a good solution for this!
Open up a plaintext editor such as CotEdit (TextEdit in macOS is NOT a plaintext editor, so do not use it for this) and type in:
open /Applications/Sonic\ Robo\ Blast\ 2\ Kart.app --args -opengl
Then save the file with the ".command" extension, which is the macOS equivalent to a .bat file. Make sure to check the box in the text editor to allow the code to be executable, and then you can launch the .command file just like any application!
4.
Making Things Pretty
Still, launching a bare command line file is pretty tacky-looking, so let's make it look like a normal app!
Start by single-clicking on the command file we just made, and hitting ⌘+I to bring up the Get Info window.
Then, go back to the Applications folder and show the package contents of the game again, but this time we want the "/Content/Resources/Srb2mac.icns" file. Drag this file onto the icon in the top-left of the Get Info window, and presto, the command file now has the same icon as the vanilla application!
If you'd like, you can copy this file to your Applications folder, and it'll look and feel just like any Mac app! (Just don't delete the vanilla SRB2K, since the command file still relies on that, silly!)
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There you have it, now the game runs exactly as it should! I hope this post was useful, and that you have fun playing! ^_^