use your part number off the pump and order a new volute housing and replace. it doesn't look overly complicated if you are mechanically inclined. there are also youtube videos on how to do this as well.
I completely agree with you Akropp.. i have a Pentair whisperflo for the water fall and it is anything but "whisper" quiet.
Knowing what i know now, I would have gone with a variable speed pump. Even for the waterfall.
I do have a Pentair VSF for the pool and its great. As Jimrahbe said...
Has anyone heard of using a good quality UV based protectant wax to keep the filter housing in good shape? During my pool school, the PB suggested applying a wax to it for protection from the sun. He said this would keep it from degrading.