Well, there is a KJW MK1 rail available, pretty sure there isn't a vast selection beyond one or two.
There is a small dovetail rail ontop of it to begin with, you can use with airgun scopes.
But in honesty, the KJW MK1 isn't all that great of a gun and definitely not too great of a sniper rifle if this is what you're intending. Hopup is incredibly inconsistant, same with the gas output and accuracy in general.
But to answer your question.
Totally depend on what you prefer, and what is available for you to put on it. I wouldn't push beyond a 4x scope, the less magnification the better for airsoft ranges.