I do understand it's a bad practice to edit other's classes so when I find something that needs fixing I always report it to the mod owner.
Problem is, as you understand, modpacks sometimes take a long time to be updated and measures like this need to be taken to avoid prejudicing the players.
I will have a look into the coremods but I do got a question regarding that:
To my understanding a coremod would be used Vanilla behaviour, involving (or not) bytecode manipulation.
How would I make it so it Overrides a class from an existent mod instead?
Some years ago I would simply ask if a simple "@Override" would do it, but this has proved to be something more complicated than I originally expected.