The engines with these problem that I've torn down, the obvious initial failure was the rear bearing cage. All the little metal shavings went up through the engine and ruined everything else
That makes sense.
In this case I'll see signs and scratches in the inside of the engine block, I'm sure they would be happy to fix it though but I rather have a new engine for that amount of money, $146 instead of having a ruined one .
I most likely ask them to send it back and use the money for a new Thunder Tiger pro 91 instead.