I've done a bit more digging into this. I had added a GPL licence to it, but I don't think that it can be truly made open source. The Copyright is still with Empire / Razorworks. From what I've read, copyright can last for 70 years, even after the copyright holder no longer exists. It seems complicated, so I thought it would be best not to mess with it.

The licence holder or holder of the intellectual property, and copyright holder seem different things.

So I've edited the original licence so that it doesn't refer to downloading a Zip file, or agreeing when you click a button. That was the method under which the original source code was distributed.

The original modders put it on a private repo so that they could ensure that modders agreed to the licence before getting access. This was to please Empire when they were still around.

But the licence actually states that Empire wished to make the source code publicly available. So having it in a private repo went against that, and having it on Github goes with the intention of the licence.

The rest of the licence seems kind of similar to the intent of the GPL licence, in that anyone can modify it, and distribute it, as long as the licence is included. The difference being that Empire could use any modifications in future releases. So it actually still serves our needs by keeping it there.