Chun, you can tell a lot about your engine by pulling the valve cover off and looking at the valve train, rockers, springs and push rods. Usually, only one or two nuts on the top of the engine hold the valve cover on. When you pull the cover, you need to keep things clean.
If things look okay, operate the compression release and slowly jack the engine over. Observe the valve train operation carefully while you do this. Check the valves and valve seats closely. Check the valve clearances as noted above and in your manual.
If I would you, I would do this step and then evaluate what to do next.
If you can take video and post it, why not do that, and perhaps some experts or experienced do it yourselfers could look at it and give you advice at that point?