Forking implies a few things which may answer your question.
First, the original author - by using a free/libre license - authorized you to fork, for whatever reason. You cannot "steal" something what has been explicitly offered for you to take.
When forking, you keep the (preferably full) list of the original authors, giving them their credit. Since you do the same free licensing, there's nothing stopping them to integrate back your changes, or to join the two projects. Or others.
Sometimes it's worth a try to contact the original people since they may be more familiar with the code, but abandonware authors rarely want to touch it again after decades. On Github there's a feature which shows you the network of forks, which sometimes cannot even display the network due to the large amount of forks around, so I may say forking is really commonplace activity nowadays and you need not to worry about it.
If the project is a useful one you will see plenty of people who will come to you and help you, or ask for your help. By forking and working on it you make the world better, more diverse (for better or worse).