Corroded Battery In Walkman

My brother has requested to borrow a CD. So whilst hunting around for it I came across his old walkman. However, I had not realised that it still contained a battery in it. The walkman is over 15 years old, and since my brother moved out around 2002 it makes the battery atleast 7 years old!

Here are some pics of the cassette walkman taken with my cheapo webcam (10!), as I don't have a digital camera. Unfortunately, the walkman was sat above an old power adaptor (wall wart) and has a nasty brownish stain on it, a bit like paint. I hope to clean and use the adaptor safely in the future :)