Ooh that tank is in nice shape. Mine is a 1976 KZ400 D3, with a '75 S1 motor in it (kick only). These bikes are not fast, but they are workhorses. In fact, it's the most reliable bike I own. Starts first (or second) kick every time. The motor I'm running now I picked up with a couple of bins full of parts for $75. Looks like hell, but I put oil in it, strapped it to the bike and now have almost 2k miles on it. Rode it down to Barber and back last fall. Did the track too!
Motor parts are also very expensive - thus the huge number of top-ends and bottom-ends on ebay... folks take them apart to fix the ever-present top-end leaks and when they find out how much it's going to cost to fix it correctly they just put the whole lot up for sale. Pistons, valves, etc are not available from Kawi anymore.
Regardless, $50 is a steal, especially if you can get one running bike out of it all. The Kawi Twins forum is helpful, but there is nowhere near the traffic that this forum gets and you have to wait a while to get answers.
Good luck!!
Mine:
