Well looks like I was wrong! I actually remember seeing that brochure but I thought it was called the "250XL" but now I see it says "KE" I guess you learn something new everyday
! The first two digits of your serial number indicate the year "70" means that you have a 1970 Cat mini bike.
As for locating a seat good luck I've never seen one of those before I've seen a decent number of the 250X seats but never one of the KE's before. Wouldn't surprise me if there may only be one or two in existence because since that bike was made for girls I'm sure WAY far less of them were sold than the ones made for boys (since the majority of the mini bike market was boys).
You would probably have better luck getting one made I have an extra 250X seat (not the correct seat for your bike but the same size) I can get you dimensions if needed and someone can try to make the pattern for you.