the existing chain will work,
undo the adjusters and carrier bolts in the back, use a pry bar or just yank up on the chain to get all the slack you can, and you should be able to undo the front sprocket bolts and slide it right off, chain and all, put the new gear in the chain and slide it back on,
but if that chain is worn beyond the specs, it will eat up that new sprocket faster than the cookie monster in a keebler factory,
it's best to replace them all (sprokets and chain) if they are worn