Geez, what a great pickup John, well done! This is my 1969 K0, same frame number as yours, but not much left of the original bike sadly. Luckily I've collected all of the hard to find parts, although I've only got a single cut fender and non-wrinkle tank, I didn't realise that the double cut fenders and wrinkle tanks were still being used 5000 bikes after the last of the sandcasts.
You can buy a repro air box and HM300 pipes from Yamiya, excellent quality, but not cheap, but considering the deal you got on your bike, they'd only add to it's value, I'd reckon? Cheers, Terry.