I would benefit from a few minor modifications to the engine, however I am using the Editor and it doesn’t have a “project-copy” of the engine code (as far as I can tell anyways).
Is there a way to have custom modifications to the engine while using the Editor ?
While actually in the Editor you have to use the engine as is. There is no way of running code while you are editing.
In the game, you can relatively easily “patch” existing parts of the engine, by including code in your scripts which replaces the parts you don’t need.
If you think this is something that other users would benefit from be sure to mention it over on github and possibly submit a pull request.