Hi Webb
Al (Metaltriumph) has covered most of the issues. The main reason you can't just 'fit up' a really wide rear rim is the chain alignment. There is not enough clearance between the tyre and the chain run.
What you can do though - is lace up a slightly wider rim to your standard rear hub, and fit a slightly wider rear tyre. The standard rear rim width on 650 zeddies is a 2.15 x 18, running a 110/80 or 120/80 x 18 tyre. You could change this to a 3 x 18 and fit a 130 section tyre - or possibly even a 3.50 x 18 and fit a 140 section tyre. Running a 140-section rear tyre would require a 530 chain. If your bike is currently using a 630 chain, you can change the sprockets to early 650 (76-78) sprockets, these are 530.
I doubt you could go much wider than a 140 section tyre. The width of the swingarm at the front is one of the determining factors.
There are, as a general rule - no 'easily' interchangeable front & rear wheels to make this job easy. There is always going to be a degree of modifiction required no matter what wheel/tyre combo you want to use. What you could try though - is checking the specs from the wheels off a 750 Zephyr. Kawasaki made this bike with wire wheels for a couple of models. One of the issues with using late-model bits is that things like axle diameters are normally larger - which always creates problems when you want to use standard brakes or even standard swingarms - you have to muck around with different spec wheel bearings and perhaps slight machining to the swingarm etc.
You can use a set of forks off any bike you want - if you can source a complete front & rear ends from a donor bike, then just do the machining to make these fit your frame. It is difficult to fit a wider front wheel to a set of standard forks, because you run into problems with clearance for the brakes etc.
Nothing ever just *bolts right up*. It always takes some considerable effort when you change things from stock, unless the changes are only minor.
Good luck