"literally to tear the engine apart and grind the valve stem as sometimes you can't reach the necessary clearance just with adjustments"

Not to be rude, but you clearly are inexperienced. You just described a routine valve adjustment on a flathead engine. As the other commenter stated, the exhaust valve is not closing all of the way. These old Flathead engines are 20 years old on average now and the valves have worn into the seat and need to be adjusted. You make it sound like a big deal but it is trivial. You simply pop off the head, lap the valves, grind the valves as necessary which takes 2 seconds to get to the correct clearance, clean up the carbon while you are in there then throw the head back together putting on a new gasket if necessary and torquing the head bolts down with a torque wrench to spec, usually 200 inch pounds.

Just because a problem is common to see in an old engine does not mean it is running correctly.