Also, has it got to be 100% aligned with the existing hole in the disk and is it really needed seeing as i'm only going to be reading the data off it to get the stream files then it won't be put in a drive again.
However, the hub rings are not really needed. The hub rings were included on most later 5.25" low-density media to address some kind of pinching or gripping issue with a few specific 5.25" drives. Unless you happen to have one of those drives, it will work fine without it. And even if you did, it wouldn't matter for just a couple of uses. In fact, I am looking at a couple of earlier low-density 5.25" disks right now that were manufactured without a hub ring.
So unless you are trying to restore a disk to original cosmetic condition, you should not bother. And even then, you risk getting excess glue all over the disk or getting the hub ring permanently non-centered.
BTW, if that disk is as grungy as it sounds there is very likely gunk in the jacket lining. If you aren't moving the cookie to a clean jacket, you might consider the cleaning method described here: viewtopic.php?f=10&t=1135&p=10491#p10491
Anyway, this disk is so bad that after cleaning both sides the mold has adhered to the surface so much that in one spot the magnetic coating has come away so you can see through the disk if you hold it up to the light! Oviousky been stored in very bad conditions and so the disk is not worth salvaging, so looking to get a replacement disk if possible instead!
I think the worst I have encountered so far were disks with seemingly small scratches that "hemorrhaged" brown magnetic particles when cleaned with a q-tip and alcohol, and some Wabash disks with something seemingly sticky on the surface that caused the drive head to pull off all of the magnetic stuff (and turn see-through) unless there were washed first.
3½" disks can withstand a gentle wiping, not damaged a disk this way yet!
If there are signs that the disk surface is coming off try to avoid using anything with alcohol/solvents in it for cleaning. In such cases it's best to cut open the disk for cleaning under running water if at all possible.
Missing center ring hasn't really been an issue here. Had only one disk where it kept slipping unless the spindle hub was pressed down manually.
So it looks like the surface is too far gone with mold to actually read without damaging the dive, so have put the disk to bed and won't bother with it again!