my old style freak back won't fit inside my sterling pump handle. You would just have to sand the handle out a little bit to make it work well.
You could get the stock barrel bored for freak inserts. Or find a nice ported barrel of whatever flavor you want and get it bored as well. I would think that would be more economical than buying a new freak kit, but don't know what prices you are looking at where you are.