Can't fix it. ABS makes the brakes spongy. You can pitch the ABS and run straight brake lines if you want, I don't know about the safety or what codes might be thrown.
I have two ABS bikes, I dislike it. It kicks in way too soon. KIBS is much better than the systems I'm talking about (F800GS BMW which I disconnected the system electrically) but any ABS is going to mess up the feel of the brakes. Even on the new R1 Don Canet was stopping faster with it turned off, Cycle World road test.
My ZX-10R 4G is non-ABS. I give the brakes a 9 for street use, both front/rear. Stock lines and pads. They have to bed in, with stock pads takes 4000 to 5000 miles.
My L3 Gixxer 750, those brakes were not very good. I had to replace the pads with Galfer HH and Goodridge S/S lines. Still took 3000 miles to bed in (street use) but they are pretty good now. 9 overall.
As you know rear brake should not be very powerful on this kind of bike.