Well you have this option. It would be a whole new seat and SHOULD fit. The bike was listed as a cb750KZ. Now I cant guarantee it but the 77/78 K bikes I think used the same seat mounting. yours is a 79 but the same rule I would think, follows.
http://www.davidsilverspares.co.uk/parts/part_29599/This url should get you to the page. It is a little pricey but not too bad considering it comes from teh UK.
Forgot to add, if you choose not to get the seat, you will need to look around for places that sell foam. Get a fairly stiff foam, remember, you butt, not your hand is sitting on the foam!
Then cut it to fit and either get a cover or use the marine vinyl.
The ride IS the adventure. The destination is just to get gas!