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.htmAgain, 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.