Re: stripped spark plug thread

[Don]

I am the minority here, would recommend finding another head instead of helicoil/inserts, improper installation or compatibility could prevent the spark plug from dissipating heat properly and preignition.
I’m not sure I understand your second question, correct me if I misinterpret. But, you will need to remove the powerhead from the exhaust leg in order to split the crankcase and replace the oring around the upper bearing. Just seems like an oring failure is unlikely unless someone has had the powerhead apart and misassembled/forgot to install the oring.
Have you checked the crankshaft for sideways slop, perhaps the upper bearing is worn/rusted.