There is no spec on valve guide clearance, Briggs used a go-no go gauge to determine whether it needed to be re bushed.
Be aware that air cooled engine tolerances are much wider than those for water cooled. If the valve guide clearance on yours were set up like an auto engine, the valve would seize once hot.Sporadic running is not usually a compression problem but hard starting and low power would be.
Have you tried an inline spark tester while the engine is running? If the spark turns yellow or momentarily flickers, the ignition is the culprit. Does your engine still have points, they are hidden behind the flywheel? If so, cut the wires and install a breakerless coil. If everything else is good you will experience easier starting and smoother running.

Hi Canguy, Great Idea with the inline spark test, I'll check the spark asap. I do have an Ignition trak tester that tests under a pressure, simulating a cylinders pressure. It has a dial with 3 colors red, yellow, and green. To test: you start in the green or highest pressure, and crank watching the 2 electrodes for the spark, and its color. There is a reference chart that tells you what is faulty or bad. In the green, if there is no spark, you go down in pressure to the yellow, and try the test again. You do this until you get a spark, and note its color. The Idea here is to simulate the cyl. pressure maintaining a good spark at the highest pressure. Then simply check with the chart to see what the defect is. IT Works, I used this on my son's motorcycle Ign. system, and it determined the coil was weak. Replaced it, and he was Good to go. Later, Nitrojc.

Canguy, This is a follow up. I bought an in-line spark tester, and did the test. At engine idle, and above I got a bright Orange light (color of test lite) that was steady, not missing a beat. So, I am presuming the Ign. is OK. NO Yellow light. Thanks, Nitrojc.