I can't guarantee that it's the valves but it would be the most likely thing to do next.
What you do is remove the valve cover and then rotate the engine over by hand to Top Dead Center, (TDC). You will probably have to remove the starter housing or the flywheel shroud to be able to turn the flywheel. TDC can be found by sticking something like a wooden dowel into the spark plug hole so that you can use this to gauge the height of the piston. Do not stick anything into the plug hole unless the piston is near the top or you will get it jammed up and cause yourself problems. TDC is defined as the very highest position of the piston after the intake valve closes. So if you've not done anything like this before, rotate the engine over by hand a few times to get the idea and then when the intake valve closes and the piston is at its highest, you're at TDC.
On the following, you only want to do one valve at a time. So completely finish one before starting on the other because you don't want to mix up parts.
Before you loosen the nuts, use the feeler gauge and check what the clearance is. It only takes a few thousandths to cause a problem. Remove the lock nut and the retaining nut and the rocker. Then pull the pushrod out and roll it on a nice flat surface. Glass is perfect. If it isn't perfectly straight, it will need to be replaced. Look at the rocker very carefully and be sure there aren't any cracks in it. Same for the nuts. Check that the stud is straight and tight. Don't try to remove it, just eyeball it or put a straightedge alongside it. Look at the valve springs and see that they look good and have good tension. Look everything over and pay attention for any spots that look burnt.
What you do then is replace everything on the rocker arm stud and insert a .006" feeler gauge between the rocker and the tip of the valve stem. Tighten the retaining nut down so that the feeler gauge just barely slips in and out. Leave the gauge in place and tighten down the lock nut. Repeat this on the other valve.
Rotate the engine over by hand and see that everything looks good with no binding or odd movement. Put your valve cover back on and put the plug back in, but leave the plug wire off, and try pulling on the starter. Hopefully this will have fixed things.