If you have Discord, message him on his channel; you're more likely to get a response there. I did that, too. I also created my own version of the mod, but only for personal use and on a private server.

My tip: the mod description must definitely state what you're using it for, that all rights remain with Ribera, and include a link to the original mod.

Hello Ribera, I am reaching out to you for permission to modify the Yamato Weapon mod for possible 2 things. 1 Personal use for my own world, and possibly for permission to modify it and use it on a public server called Nexus Redux. The owner of said server has asked me to try to balance it out a little to mesh with the mods that we use there. Mainly power consumption and damage. With your permission i would like to attempt this.
I am reaching out here as i cannot locate a way to direct messege you.
Thank you for your time.

I'll write to you directly now
Please update the Yamato weapon mod
Many weapons no longer work correctly.
And there are sound problems when firing the weapons

+rep WMG GO BURRRR

そういえば、リベラさんとはまだフレンド登録してませんでしたね。改めてよろしくですー。