Howdy. The ZN100 is kind of rare & I don't think any other models had the same tank. The ZN700 shares some parts with the ZN1100, but I don't know if the tank can be used. You could try a motorcycle salvage yard. I read somewhere that there is a nationwide database that some of them use, so you might look into that. I didn't see a tank on eebbaayy, but you could contact some of the sellers who have other ZN parts listed. Good luck.
some search terms. ZN1100. ZN1100-B1 or B2. ZN1100Ltd. ZN 1000. ZN 1100Ltd. Ltd1100. etc.
1981 KZ650-CSR. Competition, Sports & Road.