I'm guessing you have a KZ550LTD with the pseudo-cruiser look, given your location. If so, the rear of the frame kicks up at the back instead of laying flat, so no off-the-shelf seat will work right. I do not recommend cutting the rear loop unless you have a plan to rebuild the whole rear subframe to get the shocks tied together again. The whole frame is swaybacked so it's a bugger to get flat enough to work with the fiberglass seats. Ask me how I know, I dare you. This is a KZ440LTD, which has similar frame issues:
(before anyone gives me crap, that's an in-progress shot, many things are missing)
If by the Grace of God you have the standard KZ550, leave the seat alone because it's awesome and looks like a tiny Z1.
Cheapest option would be to de-upholster your current seat, have at it with a bread knife to level the foam, then make a whole new seat cover. That's assuming you have a sewing machine already.
Also, if you've got the LTD, putting cafe-clubmans or whatever will fubar your riding position. You need to move the pegs back near the swingarm pivot to make that work better. Then you modify the brake lever so you can step on it again, and make a linkage for the shifter. Do you see how this snowballs?