Stock CB750's in mint condition look great, and early ones can go for big bucks, but compared to modern bikes, they are just so boring to ride, sh1t brakes, sh1t handling, sh1t accelleration, sh1t, sh1t, sh1t. Yeah, I know, lots of folk love boring, and I can dig that too. I've got a January build CB750K0 that will stay original as long as I own it, it's just too valuable to fcuk up.
But a K5 or K6 isn't worth all that much, and often the cost of a proper restoration can be much more than making a sweet custom bike, with mods that will take it into the real world of today, with good looks, good handling, plenty of power, and good brakes to boot.
I bought this bike as a crashed wreck for 1500 bucks, the previous owner rebuilt the F2 engine with an 836cc kit, Dyna ignition and coils, and modded some 32mm CV carbs to suit. It is probably the sweetest running CB750 of the 20 or so that I've owned, I bought it "sight unseen" in that I couldn't start it or ride it until after I'd replaced all the damaged parts, so I was very pleasantly surprised indeed.
It came with a lot of the parts needed to repair the damage (dropped on the right side at approx 60 MPH) and I only spent less than 1000 bucks to get it to this condition. It's still a "work in progress" and I'll keep doing little bits as time goes by, but this bike is my new "daily rider" and I won't be selling it soon, unless something better comes along, ha ha!