Ahhh OK, I was on the wrong track. You can stop the corrosion getting worse with a coat of ACF50 which is an anti corrosion formula - you can get spray cans for around £15 and litre bottles for about £30 which will do all your bike if needed. Alternatively Nooj on this forum does a great job for a great price on a whole bike and does all the hard work for you. As for a replacement shock I would suspect others on this foruum would have a better idea than I if you're on a Jap bike.