Did I blow up my bike?

What fuel are you running?   That looks like massive lead buidup on the plug.

OK, not lead then.  Maybe some kinda buildup from sucking coolant through the head o-rings.  I'd pull the head and see what's going on.  

Yeah, you definitely blew it up.  Going to need a replate.   I'd take the bottom apart too and do a rod and mains right away.. since some debris likely got down there.

Question - what thickness base gasket were you running?  Or did you have a known squish measurement?  It looks like the piston might have been getting pretty close to the head, but hard to say since it jumbled some debris around.

Some of the debris on the head looks flattened, he can measure the squish...

I wouldn't trust that debris as a squish measurement.   It's not exactly a "controlled" squeeze..lol     Stock, they come with a fairy large squish so it's unlikely it was too tight.  They typically have 2 base gaskets, commonly a .50mm and a .15 or .20 mm.   You can measure them with a caliper when you take it apart to get a pretty good idea what was in it.  They don't really compress all that much after being installed.

Was there any indication of detonation when you were riding?  Hear any noise or anything?    Was that plug in the head pic brand new before the parking lot ride?

Piston washed clean and cloudy coolant. Before tearing down, maybe do a coolant system pressure test?

To lean or no oil or the wrong oil. 

40 degrees ambient temp would likely necessitate a jetting adjustment to richer settings….fuel type? Old fuel? Leaned out & squeaked it….

Possible…running very lean, overheated, blew head oring, coolant into combustion chamber, piston breakage…whenever you ride in an extreme cold climate, especially a high revving 125…richen needle and main by at least 1 step and work you way back to leaner if needed….

As said above, likely a coolant/lubrication issue.  New cylinder plating and top end parts, and as said, you'll need to make sure the exploding piston parts did not damage or contaminate the lower end bearings.  Sorry.  (But be glad it's not a 4-stroke or it would be 2-3 times the expense to fix)!

Yes, the rod should have side clearance and no vertical play.  There is a spec to measure the max side clearance with a feeler gauge.

I would probably get a new rod anyway. The 2022 stock rods had issues, and I would hate to rebuild it with the old rod and then blow it up 10 hours later. I don't know what rods are good for that engine right now though.

ProX or Wossner.

How long did you warm the bike up in those colder temps ?

KTM supplied 2 different radiator blockers for those bikes. One fully blocks and the other one has a small window. I know no one uses those, but if I remember correctly KTM will tell you to block the whole radiator for 40f. They even tell you to use the windowed blocker at 60f I think. Note that the blockers only go on one rad. The other stays fully unblocked.

This