maintainability – How bad is the following code

I am extending and maintaining customized extension of OpenAPI Generator.
It’s a working solution. What I am concerned here is the quality of the underlying open source project:

  1. Missing documentation of the code (considering its used as an extendable library/open source)
  2. In general structure is good but crucial, important classes lack implementation patterns or clean code principles.
  3. There are 1.8k open issues

When I have to extend some logic regarding my custom imports logic or grouping operations it just gives me headache trying to see all the puzzles and pieces.

The question

Would you consider following code bad, hard to maintain?

DefaultGenerator.java

DefaultCodegen.java

I mean wouldn’t be practical to refactor those classes, to make it easier to comprehend the logic and ease up the maintainability of it, especially as there are so many contributors who spend time on it? Imagine someone invents a new type of schema, and you go and add another else if statement in lines 1600, 2000, 2400, 4900, 5900 and 6000 something.

PS:
I don’t want to offend anyone, the code generator is actually working and being used by the thousands of people.
Chapeau to all of the contributors, especially core ones for amazing work done!