I am not finding specifications for tires needed for this bike. It appears both front and back are tubed tires. It appears the front is 2.75 width - will 80/90 work? Now, the rim size is what is getting me. I think I measured 18 inch (maybe I need to redo). But when I looked at motorcycle super center, I can only find either 17 inch or 19 inch tires in 80/90.
Back tire is giving me even more fits. It is 4.6 - 16 inch rim. I think a 110/90 - 16 should fit. But where can I find a tubed variety?! This is my first time dealing with tubed tires. Do you replace both tube AND tire? Or just tire? Tire is NOT loosing pressure, but front and rear are getting worn.

Awaz,
 First thing, does your bike have spoke wheels or cast wheels?? If they are spoke then you will need tubes. If they are cast aluminum than they are tubeless and only need to have tubeless valve stems installed to get them back to original condition.
I have seen pictures of the 80 model KZ 250 LTD with spoke and cast wheels so they must have come with both.
Now as far as tires, go to oldbikebarn.com, they have several listings of tires for the 80 model KZ250 LTD. The front tire is a
90/90-18" & the rear is a 120/90-16". Hope this helps.

I believe both wheels are spoke - hence tube. I will check out oldbikebarn.com

Question though is, do I replace the tubes as well when I replace the tires?

Also, some tubeless tires can be run with tubes.

Go here, look for Michelin Gazelle's, M62? I think is the name of the tire. You should fine the right sizes.

http://www.denniskirk.com/jsp/common/Frontpage.jsp

Almost all wire-spoke wheels require tubes.  Some early cast motorcycle wheels also require tubes.  Many of the wheels were marked "TUBELESS".  If yours are not marked, either run tubes or check with a KZ forum.

You should check the tubes.  If you are unsure of the date or quality, replace them.  Tubless tires can run with tubes in them, especially on a low-performance application. 

Here is a link to a conversion chart:  http://www.webbikeworld.com/Motorcycle-tires/tire-data.htm
Again, check with the KZ forums, but it looks like 90/90-18 & 120/90-16 would fit.  You might be able to go with a 130/90-16 on the rear if there is clearance.  The old bikes had really skinny rims, so you don't want to go too wide on the tires.

edit:  Noticed that size question had already been answered. 
Bolded part about replacing the tubes.

Thanks for all the info. But I still have one un-answered question. When I do find my tire of choice, do I buy BOTH tube and tire? Or just tire and use EXISTING tube?

Replace the tube. Tubes for that size cost, what, maybe $10 depending on make, replace the tubes. Don't forget the tube strips too.

Thanks for the answer Strawboss. And thanks everyone else for their response. Guess I will be tire hunt tonight.