Check the voltage at the small wires going to the coils. This should be the same as battery voltage (with the engine off) if it is less than battery voltage then you have a problem with the aged wiring.
A website
wiredgeorge motorcycle carburetors - Vintage Japanese Motorcycle Carburetor Sales, Rebuilding and Restoration - Home covers a modification using a relay bypassing the wiring. His website lists parts and wiring diagrams. Many riders at
KZrider.com - Home have done this mod with results that make their bikes run better than new. Give his website a look over and make your choice.