I recently had this same problem with an older BC where the junky plastic that was once common for shoulder ports became brittle with age and broke, but the rest of the jacket is in excellent condition. Here's what I've found so far.
You may be able to find someone to fix this for you (I haven't taken this route), or you can try yourself. On that latter score, there's good news and bad news, depending on what size your port is. I'm going to say there are two main port sizes, though there could be more. Unfortunately, I think you have the larger sized port, like I do.
There is a replacement small-sized port readily available
online for under $10. You need the inner and outer port parts. You'd have to carefully remove, or at least trim and smooth down, the portion of the old port that's still welded to your jacket. After that, the inner port fits inside the jacket, and the jacket material around the hole is sandwiched between the inner and outer port halves. A special tool is needed to screw the two halves tightly together (again, I believe there are two sizes, for the two different port sizes), and you might need a little sealant on one or both sides of the jacket fabric to make an airtight seal.
Unfortunately, I don't believe there is a similar version of the large port available. Going from memory, the small ports are about 1.75" OD across the male threads, the large ports are a little over 2".
If you need the larger port,
here is a good, but expensive ($80) repair solution.
However, if you have the larger size port, don't want to use the Scuba-Fix solution, but still want to tackle this yourself, it can still probably be done. I say "probably" because I've obtained the two-part large port, but haven't yet tried to fix the jacket. It turns out, some vendors used two-part large ports on their (mostly older, that I can tell) BCs. Some of the older SeaQuest (the one I obtained), and I believe SeaTec jackets used these removable two-part ports. You'd have to scour eBay to find one, and they are not the most common variant or the easiest to spot. I had to contact several sellers and ask for pics and that they inspect the port, before I found a style that had the two-part port. The good news on that front is that the separate inner bladder used in that older BC tended to rot as well, so you're dealing with a seller that has a mostly useless item.
As a further comment on the practical difficulties, it's quite possible you'll find that even with the larger two-part port, which is nevertheless the correct size port for your inflator assembly, the flange on the inner half does not overlap adequately with the jacket material (once the old port remnants are trimmed away) to make a good seal, and you'll have to glue a piece of fabric or other material to the existing hole, into which you've cut a slightly smaller diameter hole for the inner port half. In eyeballing mine, that's how it looked to me. If adding additional material is needed, you may have already realized that simply using that effort to adapt your BC to the smaller sized port, would allow use of the more readily available replacement port. Of course, you'd also need to swap to the matching inflator hose assembly.
All in all, unless the jacket still has high value to you, or you like these kinds of projects, you're probably better off to just write it off.
