To my knowledge nobody has successdully done a retrofit. I think there are multiple issues, not just the mounting holes but also I believe the new backplane would not line up with the old drive cages.
A few questions for you would be;
1 What version(s) of older S45s do you have? Do they have direct physical wiring to every drive slot, or do they have an array of five-drive port multiplier backplanes?
2 What is your goal in “modernizing”? IE, do you want a specific SATA/SAS speed/technology/connector type? to be able to use a specific HBA? Depending on what hardware you have and your goals “modernizing” might just be replacing the motherboard and HighPoint Rocket 750s and not touching the drive wiring.
You can look through this thread Modding/Upgrading BackBlaze Storage Pod 2.0. Your pods may not be as old as 2.0. so the stuff at the beginning might not be relevant, but you can jump down to message 79 or so where a backplane was attempted to be designed and failed. Not sure if that work is open source and could be picked up and continued.
There is also this A Disk Shelf Project. Although you aren’t trying to make a disk shelf, it may provide some resources if you wanted to try to re-do the whole drive cage area with backplanes purchased from 45Drives or using the youtuber’s design (since it apparantly works?) and printed and assembled via PCBWay or your other low volume PCB manufacturer of choice.