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I didn't see you are the original owner. That is also the least likely possibility. Just trying to get across it may not be your fault. Could even be a bad batch of pins or bearings.

I’m just not seeing the $4k price tag, if you replace all top end parts from crank up. Did you mention that both cases are cracked? Assuming that you’re gonna do the work yourself, and that both cases aren’t cracked, and using aftermarket parts, the cost might run close to $1400-1600.

Little difference between 125 and 250/300 maintenance?? If you are talking filters and oil sure but all other intervals are way different, IMO

That puppy sure went! Sorry to see.

Holy crap thats grenaded alright. Looks very dry for being the bottom end

I definitely don't dispute that. Running 32:1 vs "recommended" 60:1 means it was running at a leaner air to fuel ratio than intended. OP said "all stock", so no jetting changes made to compensate? Not sure how good the stock jetting is on these bikes though... For all I know it was dialed with the premix ratio change. Either way, feels like an air leak would be the most likely culprit as mentioned above. Or, the OEM cranks are occasionally garbage.

I hate to agree with @MxAddic but the small end looks burnt, overheated, from the blackening of the base gasket, I'd guess an air leak through the base gasket, causing it to run lean.

At some point it got hot or dried out. The chance of a good bearing failure are slim at best.

100 hours is nothing as long as you don't buy into the 60:1 and leaner BS for the 250s and 300s Jet for 30-40:1 and your crank will be happy for a couple hundred hours...

Would it be cheaper just to get a new crate engine? lol

Wow, that sucks. Looking at that failure, even if it had an air leak, stuck a piston, or wrist pin failure, there is no way that the head should have failed like that. At most it’s gonna just seize up and maybe blow the head or base gasket. It’s a clear manufacturer failure. I guess it doesn’t do any good now unfortunately. Just crazy 🤔