The PW80 came with a 32/15 set-up. This should use an 84 link #420 chain.
Chain length caluculation is a bit much for most to tackle, and you would more than likely fall between reels anyway after you figure it.
Buy the #420 chain with 120 links and use a chain breaker to make it the exact size you want. 120 links will be plenty, just wrap it on the new sprocket set-up and you will see where it needs to be...be careful. Also, have the chain adjusters ran all the way in when you size it.
Many chain sellers these days won't cut a chain for you anyway. Nothing worse than a mini dad who bought the wrong length of pre-cut chain, for both vendor and kid.