Hmm you're probably right about the cam, I always assumed it was the ignition. I assume I need to pull the motor if I want to change that now.
It is do-able with the engine in the frame, but complicated, sorta like this:
1. loosen all 4 carbs' air intake pipes, or just remove the 2 airbox bolts so it can move a little forward.
2. Remove the exhaust pipe(s).
3. Pull all the engine mount bolts except the bottom rear one: loosen it instead.
4. You might have to loosen the drive chain if it has less than 1.5" slack midway between the sprockets. Otherwise the engine won't pivot downward very far.
5. Remove the [center top] breather cover.
6. Remove the cam cover's screws (BTW: this is one reason why so many racers used Allen-head screws in the cam covers...) and the breather hose.
7. Remove the sparkplugs (makes the engine easier to turn by hand).
8. Finagle the cam cover upward and forward: it should let you tilt it toward the front. This is/was how we changed cam cover gaskets that leaked (K0/1 engines mostly). This will give you [tedious] access to the 2 cam sprocket bolts.
If/when you turn the engine to get to the other cam sprocket bolt, DON'T turn the engine backward with the big nut by the points, or you will bend the shaft the spark advancer is mounted to. This makes the timing 'jitter' back-and-forth, seen best with a strobe timing light. Then you'll have to straighten it (again?). I just 'bump' the engine with the electric start or use the kickstarter if possible, the alternator rotor bolt is the best bet (but then you're also taking off the alternator cover to get to it).
Easy peasy? Beats pulling the engine, but it's still most of the same work! This is why the 'frame kits' became popular.