Corroded battery contacts.

Hi folks,
Just looking for some advice on how to replace the negative battery contact on my Yamaha APX-5A. It just fell out the other day and is completely blue and flakey. I cannot find the screw anywhere and presume it has completely eroded. The positive contact is fine though and appears unaffected. I didn't notice any problems until recently as I lent it to a friend while I was away for a year. Is it as simple as finding a screw and a bit of copper or would I need something really specific to avoid melting the inside completely. Or will I have to replace the entire fitting? Any advice would be greatly appreciated.
Cheers.