TOPIC:

With my KZ440 LTD being turned into a scrambler, I want to put some fairly aggressive tires on it.
The front being a 100/90-19 is not a problem. However the rear being a 130/90-16 is making tire choices extremely limited. 
There is a set of 18" mag wheels(and swingarm) from a 1980 KZ1000 for dirt cheap on FB marketplace. I'm wondering if anyone knows if the rear wheel(& perhaps swingarm) will work on the KZ440.

If that isnt an option, any what would be the next best tire size for the 16" wheel?

As long as the axle diameter is the same and you are prepared to make custom spacers if necessary I don't see any reason why the 18 couldn't. Depending on the length the swingarm may make it slow to turn if you extend the wheelbase ?

If the 440 rear wheel uses a drum brake, it will be a bit of a project to convert to a disc rear wheel.  

Hmm some good points. The brake is the biggest issue, although I would love a rear disk break.

The rear brake on the 440 is small. The bigger models w drum have larger components.
I can't imagine an easy disk swap but what do I know. I'd guess you've have to swap swing arms if it can be done,
then that leads to other problems such as drive chain alignment.  

A bigger motorcycle would leave you with more options.
My 650-CSR has the same frame as the 750-Ltd, so a disk conversion would be a simple matter.
Same with the front end >> the stuff from a 750-4 model (especially the Ltd) would be an easy Fr dual disk conversion.
I put 550-Ltd cast alloy wheels on my csr and that was easy >> everything was a direct bolt-on since most of the components were identical or virtually identical.