Front calipers are dragging

If the brake fluid has not been properly maintained (as in changed on a regular basis) you may be in for a nasty surprise. Gloppy stuff can form in the MC, calipers, and brake lines, and that will affect brake performance. Regular maintenance will prevent this. I would start by flushing the brake system very thoroughly with the proper brake fluid; that may solve the problem. Ed

Yes, first change the fluid, then be sure what you are feeling is really drag, and not just normal. Of course all disc brakes have a little drag, some worse than others and some slight drag should be considered normal. If you jack the wheel up and spin it by hand it should not be hard to turn, but isn't going to spin 100% free either. I never had any problems with KZ calipers, but new motorcycles recommend silicon grease for slide pins ect. Napa has a good silicon grease. Much better choice than lithium.
www.napaonline.com/napa/en/p/BK_7651351

What's the best way to clean out the lines and return holes?

Unless they look like they have butter in them, fresh fluid is the thing to use. You don't want brake cleaner on rubber patrs.
Steve

Use clean brake fluid to flush out the old stuff. Ed

I wouldn't even bother trying to flush out the system. You got chunks of coagulated brake fluid in the caliper and master which is why the system is binding up, and the inside of the brake lines are coated with scale too. You can NOT flush this sort of crap though the system. It won't work.

This photo below is of the inside of my old 750E brake, and the caliper photo shows a typical caliper after 30 years on the road without proper maintenance.