Definitely remove the old cover. Replace the foam, if necessary. You should be able to carve and shape it yourself using an electric carving knife and disk sander. Here's a picture of mine in the works. The black lines are the references for where to measure the height of the foam based on the upstand of the cover at the corresponding position. Basically, started out pretty thick and worked it down based on trial fits. Once there, a layer of dacron to smooth any lumps.
Also, a picture of the finished seat on my bike.
Richard L