You can just replace just the cam but I am sure you realize that it won't fix other parts of the engine.
I don't remember the story right now but with that era bike I would research the stock cam timing. It was advanced 3 degrees in '78 I believe and I think they did it with the cam sprocket key slot. 336's now come advanced 3 degrees by the cam's key slot. It might be possible to end up advancing a 336 by 6 degrees with the right combo of parts. That would not be good.
Always check you lifters every time your cylinders are off. Replace just the flaked ones. I did that 65,000 miles ago and now all my lifters are perfect. They have tested good and are staying in there.