I would replace the handlebars first. With flatter bars you may need a different master cylinder & long stem mirrors.
Emgo makes a number of handlebar controls. Z1Enterprises.com carries a couple of them that are stock type & they might be able to get any of the others.
For shorter control cables I would try to get oem from a standard model, such as a 750E/R/L or KZ700. from the Kawasaki dealer or eebbaayy. Z1Enterprises.com may list the measurements for comparison.
I installed Goodridge brake lines & fittings from Z1Entprises.com. Cyclebrakes.com has Galfer brake lines which have the fittings crimped on. There's less chance of leaks, but I haven't had any problems having separate fittings. With removable fittings, you can swap out to a different angle if necessary for whatever reason.
1981 KZ650-CSR. Competition, Sports & Road.