Old Japanese bikes are usually easy to work on, and the parts for the '70s ones are usually easy to find online. Try SCI on ebay. They usually have tons of stuff for older machines. My friend just got points, condenser, seals, and a carb rebuild kit for his '66 Honda 160. I'm pretty sure you will find what you need for your '77.
The seat is a different story. There is always a possibility that you will fine something in a salvage yard that will work for you, but that's a long shot. Plus, the bike was designed to have a long seat. Anything shorter will leave stuff exposed that wasn't meant to be seen.
I have seen plenty of cafe racers that have the stock seat foam cut down and the cover trimmed. It's actually a pretty cool look, but it will only work for certain shapes of seat, and I don't know about your Kaw.