It sounds like maybe you were running a 70cc kit with a pipe and a small bore single circuit, potentially under jetted, carb at low throttle which could lead to a lean mixture, less cooling (less airflow , less charge cooling), and less lubrication than ideal. Also, you put a bunch of miles on it with a massive airleak, most likely causing lean condition overheating damage.
If you got the highest quality kit money could buy, and all these things happened again, what do you think would happen?
What did your plug chops look like? at what jetting were you four stroking? What were your cylinder head temps?