I use a pistol scope scout mounted like alot of people recommend on the Crosman forum. It works well, easier to pump and load then with the scope over the breech.
Otherwise, recommended using the Williams receiver sight if you can. If not, there are several barrel mounts for scopes, two part and one part. Mac1 Airguns sells the best one. The two set mount is cheaper and can be had more commonly, try Pyramyd Air.
Still, the scout mount is preferred.