I know most people will probably disagree with this, but I don't care. It's my personal opinion that when someone leaves the modding scene, and remains unable to be contacted (because they don't care anymore), their stuff is up for grabs. Credit is still owed, of course, but if you can't reach someone who's quit modding, you won't hear any complaints from me about using their stuff.
Other people feel very differently though, so I'd tread with caution. With any luck, you'll get a response before it's time to release your mod, and thus be spared from the inevitable gripes.