If you don't want to go to the expense of having the pistol professionally refinished, remove as much of the rust etc. as you can, then degrease the surface, and apply a coat of Black Acrylic spray paint which will give a good hard wearing finish. The use of the cocking aid will always cause some wear to the fore end, but this can easily be touched up as necessary.
You could use either semi matt or gloss according to your preference, but I have found the satin finish, available from Halfords and probably elsewhere, to be a pleasing finish.
Metal polish is good for removing marks and scratches from the plastic.
I have a few Scorpions both boxed and unboxed, but I don't have a Cougar.