Here is an overview of the "big four" Kawasaki Z bike. This covers the Z1 > KZ900 > KZ1000 > KZ1100 > GPz1100
From the highest level, you can split these up into two designs. The 1973 to 1980 bikes, and the 1981 to 1984 bikes. The Z1 was tweaked and refined until 1980. For 1981, nearly everything was redesigned. The frame, engine, brakes, etc are substantially different between the two groups.
The first group includes the Z1, Kz900, and Kz1000. The Kz1000 had several trim variants. The standard "A", the LTD (
, the Z1-R (D), the Shaft (E), the Police (C), Classic (G). All of these years and models shared the same general frame design and engine design. Parts evolved over time, but they are more similar than different. The trim differences are largely cosmetic, primarily the handlebars, seat, fenders, tank, and exhaust, although there are some real mechanical differences between trims. The Shaft and the Classic are probably the most different mechanically.
As a rule, the engine is nearly identical across all trims for a given year. The engine was revised for 79-80, called the Mk2 engine. There were real changes to the head, crank, cases, etc., but the parts are nearly 100% swapable between Mk1 and Mk2 so it's still pretty much the same engine.
The second group is the 1981 to 1984 (but actually 2005 for the Police model). A clear division within this group would be the twin shock and mono-shock bikes. The GPz model for 83-84 had more power (better head), plastic fairings, the mono-shock, fuel injection, etc. The (non-GPz) twin shock models are more similar that different. Twin shock trims included the CSR, LTD, Spectre, Shaft, ELR (Eddy Lawson Replica), the standard "J" model, Police model.
The difference between the early 73-80 and 81- bikes is substantial. Nearly every part was re-engineered. The frame is all new and substantially stiffer, with new geometry. The engine is an evolution of the '73 design, but nearly ever part was redesigned. The new for 81 engine is typically called the "J" engine. They don't easily swap between frames, as the "J" has different mounts to add rubber isolation. Continuing with the changes, the brakes changed, the wheels changed (tubeless), the gauges changed (more like a dashboard), the carbs changed (constant velocity instead of round slides). Really nearly everything was revised.
Here is the model and trim evolution of Z1. The Z1 ran from 1973 to 1975. The Kz900 appeared in '76, but was nearly the same. The LTD sub-model appeared in '76. The Kz1000 appeared in 1977, along with the Police model. The Z1-R variant appeared for '78. The Mk2 engine appeared in '79, along with the Shaft model. The fuel injected Classic came in 1980.
I started a list of all of the detailed model changes from 1973 to 1980. I'll can add it if anyone wants. Someone else will need to do the >81 bikes, as those are not in my head.