What size tires?
For instance, in the 400-18 and 3.50-18 sizes, Sirius Consolidated has Duro Tires that look like the period correct Dunlops. I used Dunlops in 1972 on my CB350, and the Duros look very much like them. The Dunlops at that time were the "State of the Art" tire. Not so much anymore, but the handling was a big improvement from the older style tires that pre-dated them. To me, the Firestones look more 1950's. In the late 60's, the Firestone look was just old and crappy technology. The look fat (or is it Phat?), so I understand the draw, but from a practical handling and performance aspect, they really aren't all that good. The tread pattern and shape are still basically 1950's understanding of the tire. the rubber has improved, but large, soft sidewalls aren't conducive to handling, but mostly to a soft ride. Cars have had the same transformation. If you look at 1950 car tires, if you can find any, and compare to a modern tire, the shape, structure, tread, all are vastly different. It is more than just a material change.
Look at a Dunlop K70 tire (they still make them. The have the tread pattern that was popular as the "new" thing when your bike was in production. They cost more then the Duro's, but are actual Dunlops. Your KZ never had Firestones.