I don't know if this story helps or not, but many know I've got a 561 cc big bore kit 11:1 compression and larger cam w/ otherwise stock valvetrain. I paid A LOT for it. This summer the engine got louder and louder at idle but I played it off. Then one day I smelt a whiff of oil, and I was 3 oz low and it was darker (check every 100 miles). I'm wired internally to be overly anal, just the way I am. So I stuck the bore scope down the spark plug hole and saw light scoring on right side of cylinder. Tore it down, and left side of cylinder (thrust side) and piston were gone. I mean aluminum transfer to cylinder, grooves wore in piston and stuck rings. Long story short the company I dealt with warrantied it out (very good company in my eyes - took credit for too loose bore), new jug, piston/rings, gaskets. Down 2.5 weeks, I now have about same miles on it than before and still super quiet and super performance.
My point in all this is this: I still had great power, started easy, ran fine, on this heavily modified 475 cc Honda engine that had a major internal damage but still running. I went for a trail ride right before I bore scoped it and tore it down. Obviously not too much longer and it would/could have been different. I did have some symptoms though. But before this thread I was and am still impressed with how well built these are. This is just my opinion but there's gotta be more going on, much more severe than my experience as my troubles were really no major trouble at all besides 2.5 weeks.
Either way nevertheless that much trouble should not happen and can be extremely frustrating. Regardless of the cause/fault etc. My favorite (only) thing I like about breaking down is finding the cause to fix/improve it. I hope you find yours.