Usually most of the undocumented runtime functions that you used still exist but are either named in PascalCase or have an additionnal underscore before them.
For exemple this.inst.GetWorldInfo()._SetLayer()
If a plugin has a huge class that does all of the heavy work (like rex's addons sometimes do) you can just copy paste the whole thing in plugin.js or behavior.js at the very top and expect it to work fine.
If said class uses functions from C2 runtime, just replace them but it should be easy.
In C3 runtime the "this" you're controlling is not an inst class but an sdkInst. So if you want to call a function from another plugin (like sprite) you must do
This will also let you access any property defined there, like current animation, and any defined helper function.
The easiest way to know what you can access is by using console.log on "this"
It will print every property, and list all of the functions you have in the last property named "__proto__". Each function will be named, and will list the needed arguments.
The minification scirra used for the runtime files is pretty basic. They just replaced every function and var names with a single letter. All the runtime function names and Exps, Acts, and Cnds names can't br altered though so they haven't been changed.
It's a way different story for the editor sdk though.
However, I'm pretty sure that since the editor is also JS you can console.log your way through everything as well.
I think you can get every file C3 has if you managed to host C3 on a local server. I haven't done that myself, but I know it is possible, and will let you access the addon's editor code and commonAces files.