Doddy that's a seriously big job you're contemplating! To get that background looking realistic, natural and not too uniform is going to be difficult. You're also going to have a lot of difficulty retaining the whiskers and eyelashes etc. It's a good pic as it stands, and I'm not peeing up your leg - it is! I'd leave well alone. What you could do is lose that building on the right. Other than that just use selective feathered masks to get the pic how you think it looks best. You can subdue or increase background brightness etc, to you own tastes.
I just had a quick, and I mean quick, bodge at your pic. I've lost some of those stray hairs between the ears and down the mane just for a tidier look.
