I've not flown a Kitfox, but suspect from those I've looked at that it's a basically sound aeroplane, but probably not quite as a refined a design as either the Easy Raider or the Cub. As to how refined the individual aeroplane is - well that depends upon the actual aircraft and the design isn't really the issue.
Comparing the Easy Raider and Super Cub, and I've a reasonable number of hours in each. Construction is similar, the Easy Raider is the better short field aeroplane, cheaper to run and much more fun to throw about, the Supercub is more comfortable with more elbow room, is by far the better tourer - and offers a certain olde-worlde chique.
No idea about the J3 versus PA18 Cubs, since I'm afraid I've only flown the PA18.
Where the Easy Raider or Escapade are concerned, they're both homebuilts, so the builder has the right to fit any engine they can within the power and weight limits, and I doubt that a 912 would give you a problem on either. But, I'm not sure either needs the extra power of a 912.
Incidentally, I've flown the Escapade (the twin-stick / side-by-side version) also - which gives you a choice of which end you prefer the third wheel, and much prefer the taildragger to the nosewheel version. Mainly I don't like the lack of a steerable nosewheel (the tailwheel IS steerable), which makes it a bit of a beast on the ground.
G