I would guess that you need to do a valve adjustment. The missing can be caused by valves which are too tight. Tightness is usually indication that the valves are worn out and is common because the engine was built for leaded gas.
Do a search for 'valve adjustment' for more opinions and info. Since you are brand new to this, I will do one better and explain it here.
Go purchase a replacement valve cover gasket (rubber is better than cork if you can get it) You'll also need to find a .010" and .020" feeler gauge. Almost any basic feeler gauge set will have both sizes.
Remove the valve cover.
For your first time, you will want to adjust without the motor running, and then repeat the process with the engine running. It's sometimes difficult to get the adjusters moving the first time in 30 years.
Intake valves use the .010" gauge. Observe the intake manifold is aligned with the intake valves, so you can easily see which are intake or exhaust. Notice that they are not arranged every other valve, intake/exhaust because that would lead to improper adjusting.
Removing the spark plugs will make it easy to turn the engine by hand with the fan, or you could randomly crank the starter and just keep track of which valves still need adjusted until you've got them all. It's your choice.
Select an intake valve which is in the closed position and the rocker arm should be loose enough that you can tap it back and forth a little. Now insert the .010" gauge between the rocker arm and the valve stem at the top of the spring. If there is a great deal of extra clearance, you should tighten the adjuster until the gauge slips in and out easily, but fills all of the gap. A little loose is better than too tight, so stay on the side of caution for your first time adjusting. What we are really looking for is an exhaust valve which is too tight.
Repeat the process for all of the intake valves, cranking or turning the engine in turn until each is in the closed position and adjusted. Now move to the exhaust valves and use the .020" gauge. Tight exhaust valves may have little or no clearance and will cause poor engine performanc. Check the two 'dead' cyclinders and be prepared to loosen the adjuster until the gauge fits again. The other exhaust valves might need adjusted in either direction.
Now, once you've adjusted every valve, clean up the engine bay and start the engine with the valve cover still removed. You should notice that the valves are not as loud, but some ticking is normal. You shouldn't be afraid to check your work with the engine idling. Slip the gauge into the clearance and it should move easily except for an instant while the valve opens each cycle. Check them all.
If you are feeling very confident about your work, you should allow the engine to warm up for a bit and then do a hot adjustment with the engine running. It's a little more tricky to turn the wrench with rockers moving, but once you get the feel for it, you'll be able to adjust much more quickly and accurately than the beginners method that I have described.
I think we all did it cold and not running our first time.
An adjustment might make the engine run much better for a while, but if several of the exhaust valves were too tight, then the solution will be to have the head rebuilt or replaced.
Wow that was long. Who wants to turn it into an article once and for all.
