TOPIC:

What other years of the KZs are interchangeable with a 78'?

Chassis, engine components, wheels, wiring, forks... Ect.

In general the 77-80 models are most similar to each other, having similar frames & basically the same engine, but there is variation from year-to-year & between models. You can compare parts at Kawasaki.com & Partzilla --- the latter has an x-ref. Z1Enterprises.com has maintenance & other stuff for the various Kawasaki models.

The later 650s are significantly different, but you could swap engines. Or swap a 750-4 chain drive engine for more hp. Also, you could install the electronic ignition from one of them --- you'd probably need the timing advancer from the 81-83 650

It varies depending of the specific part in question. Some parts were different between KZ650 models even within the same year, some parts were different between different year bikes, etc. To be sure, you'll need to research each part you plan to swap. Ed