You could 100% reach out to the author and ask permission to do this. In 2013 I had a rotation where I wrote one chapter a day for each of my handful of stories so they all updated regularly at reasonable but low quality. I eventually abandoned them all to write what I consider my magnum opus, the progress of which has been severely hindered by college. Looking back on my older stories I'm appalled by the quality but fully intend to return to them and clean them up once I graduate. If someone stole them and finished them in their own vision before I could do that I would be absolutely LIVID and would not hesitate to report the theft - because that's what it is. However, I have one or two I don't really care to pick up because they didn't have a clear direction and endpoint for me, anyway, and if a respectful fan wanted to adopt them I would be happy to pass it on. I'm a reader too, so I know the frustration of having your favorite stories left without endings, but you have no grounds to pick them up and finish them yourself. The only way I could see this being okay is if you wrote it for yourself and never shared it online or even irl.