Decel cable should be cheap to replace.
There's a DIY from theduck on how to fix those diaphragms. I think that's what's causing your issues. You could try gorilla glue or a bicycle tube repair kit. I'll look around for that DIY for you.
EDIT: I found the DIY for you.
http://www.kawiforums.com/ninja-250r...patch-diy.html