Licensing and redistribution

Hi all, I have modified versions of several popular mods and several server owners are interested in using these mods.

As an author myself, I understand the simple courtesy and potential license requirement of asking permission of the original mod authors. However, some of these mod authors are either AWOL or simply not responding to my inquiries. I'd like to avoid any controversy if at all possible, so I am asking the community what is our stance of mod forks and what are the license requirements, if any, for said forks?

Thank you.