should I upload a working source, even if the quality is worse?

This Post: post #4811777 now has a bad id, and I have a source for the image that still works (https://www.pixiv.net/en/artworks/78977065), but the image is a lower quality here. Reading the upload help page, it says that if the image is similar to an image that has already been posted, and the image you want to post is of lower quality, it should not be uploaded. But it doesn't mention anything about bad ids, so I am just looking for some guidance here. Should I upload it and make it a child of the post with the bad id, should I just not post it, or should I do something else?

In general, it always depends on the image itself. If the revision is of poor quality, you shouldn't upload it. In the case of an image being censored afterwards, you can most likely upload it, because a censored and an uncensored image are seen as two different images (but the uncensored one is the parent). IIRC some approvers don't approve censored images, if the uncensored is already uploaded, so it's not a 100% that the image will be approved if you upload it.
Always remember that an image has to speak for itself and if you think, that it's not good enough for danbooru standards, you can skip it.

There is no point in uploading the worst image. You can leave a comment if you think it is necessary to inform about the availability of an alternative option.
Be careful when looking for alternatives. There was a paid distribution (.patreon.) 7266x5880 9.24 MB - such downloads are prohibited.
Likewise, do not confuse censored and uncensored pictures. These are different works, as per your example above.

Just leave a comment on the post with a link to the other sources. No point uploading worse images.

Thanks for the advice, I'll leave a comment on the post linking the new source, since the new source both of lower quality (800x647 vs 1280x1036) and censored where the original is uncensored.

FYI we have quick ways to link to other sites: you can use pixiv #78977065 or twitter #20 to link to specific supported sites, for example. The same goes for danbooru posts: post #4811777.