Unable to migrate from swagger-codegen due to removed method in CodegenModelFactory

Description

I'm currently migrating from swagger-codegen 2.3.1 to openapi-generator 4.3.1 and I came into a big problem. We have a highly customized generator and templates, and I'm currently stuck on a method that was removed with https://github.com/OpenAPITools/openapi-generator/pull/1744. We use this to add more fields into Operations and Properties so that they can be used on the mustache templates.
How can I overcome this drawback?

public class MyJavaJerseyServerCodegen extends JavaJerseyServerCodegen
{
...
   public void processOpts(){
      ...
      CodegenModelFactory.setTypeMapping(CodegenModelType.OPERATION, MyCodegenOperation.class);
      CodegenModelFactory.setTypeMapping(CodegenModelType.PROPERTY, MyCodegenProperty.class);
      ...
   }
...
}

public class MyCodegenOperation extends CodegenOperation {
  public String entryPointResponseModel;
  public boolean supportError501;
  ...
  public boolean hasNonCookieHeaders;
  public boolean hasHeaders;

  public MyCodegenOperation()
  {
    super();
  }
}

openapi-generator version

4.3.1

Start by reading CodegenModelFactory and the change linked as pull request 1744, then inspect JavaJerseyServerCodegen, CodegenOperation, and the custom CodegenModel mappings shown in the issue. Determine a supported replacement for extending operations and properties in version 4.3.1, and document or test how custom fields remain available to the mustache templates.