Let's hope you found the reason it was a $1200 car.

I suspect above idle. The valve was moving faster and perhaps less able to seat as much. Much of the positioning control might have been lost. Dynamic compression perhaps fell as a result. If the valve guide was also substantially worn. When you adjusted the valves it cracked at that time. Or a crack was established that completed itself very soon after.

As long as the engine ran properly when you purchased it. There is no chance that the previous owner had tried to adjust the number two element that you seem to feel looks moved.