Hello. It is amazing that someone has managed to fix the ongoing UI issue, as for someone who enjoys the mod, and has commented here in the past, although the visual bug does not break anything gameplay wise, it still looks bad aesthetically.
That being said, I do hope this fix exists, and to find out where to get this permission I could think of two ideas, plus an extra one.
First, as he had worked closely and was SimRealist's Lead Mod Developer, this is the first place to consider as a representative, as I'm sure the mod was made while working in this position. Even though they might not "hold the licence", they might know who does.
Second, you can always try to track down and stalk this person to know exactly who is close to them and who, eventually, got hold of the rights to this mod. This, of course, comes with the price of being not really morally accepted, as you'll be stalking your way to find a person so you can ask if you can modify Victor's mod...
Which brings the third idea. The only legal issue you would realistically get is a Cease and Desist order, which would be fairly easy to remediate. You'd just have to stop sharing the mod. Honestly I do not see why they would do that to begin with, since that is not tarnishing his mod and, instead, is making it so that more people can enjoy it nowadays like it has been enjoyed in the past. I believe that a midterm solution would be to just post the link of your fix in this comment section, or tell everyone how you did it.
TL;DR: Talk to SimRealist, or try to find any relative through stalking or otherwise just bite the bullet, post it and if someone sends something your way it's at best a C&D so just comply to it, although it seems unlikely
EDIT: Apparently his brother did start a GoFundMe campaign to gather funds to sustain the family through that rough time. The campaign is still open and his brother can be contacted. Try to reach him and see if you can get to an agreement?