Is the KZ650 18" Rear Alloy the same as on KZ1000?

I've heard the bearings are different sizes and can be changed, but is the casting basically the same for 78 models, or similar, with the 4 bolt rotor?

Yes, the 650 version has smaller bearings but those can be swapped out if needed. You still need the spacers for whatever bike the wheel is going on though as they are also different.

So if the KZ1000 already has an alloy rim you would already have the correct spacers?

Yes it should but if you already have the rim why are you trying to swap in another one.

Going to an 18" rim on the rear - plus the wheels I have are Lester Mags so I plan on changing them both as I don't imagine ever paying the prices being asked for an 18" Lester Mag. I bought a front wheel today and am still deciding whether to go ahead putting my 18" Police rim on the rear, which benefits from being tubeless and 2.5" wide, but means I have to change the brake caliper for the different rotor or whether to pick up a KZ650 wheel that's for sale locally which has the right rotor fitment etc. but is only be a 2,15 rim.

They are all 2.15" rims, the only one that was different were the KZ1000Rand KZ1000P, they came with 2.50" rims

Id try and get the other wheel complete with all the spacers so you can trial fit everything and make sure the wheel is centered and caliper isnt in a bind.

At the moment I have a 1988 Police Wheel which is the 2.5" width. I think this is better suited to a 120 tire. It looks, and from rough measurements, to be the same width as the current rear with regards to the spindle, I will trial fit it here in the next day or so. Biggest issue so far is the rotor it's the 7 bolt type not the 4 bolt I have two front GPz 750 rotors that bolt up to the wheel but they are 260mm diameter. I have not been able to find a rotor with the diameter of the original and a 7 bolt mount. The GPz550 has a 260mm rear rotor, if the parts diagrams are to be believed,, I bought a GPz rear caliper off ebay, I'm going to see if it will fit, but suspect the diameter of the hole may be too small for the spindle and wonder if it will be the same width as the KZ1000 one from a spacing point of view on the spindle.. Given all the complications, I'm wondering about getting a KZ650c rear wheel, which seem fairly common. that would accept my existing rotor.