Engine of barn find-steps to evaluate/save

I agree with removing the valve cover. Then, just LOOK at all of the valves and see if there are any "short". That would indicate a stuck valve and might be what the piston is coming up against. If you find one, take some GOOD penetrating oil and spray into that spring and try to SOAK the valve stem where it goes into the guide. Let it sit a while and then come back, loosen the rocker adjuster, remove the pushrod (probably bent), push the rocker out of the way and tap the end of the valve stem and see if you can free up that valve. Tap on the VALVE TIP, and NOT the valve spring retainer.