Motorcycle Repair: KX65 white smoke and locked up rear tire, little beast, main bearing

Question
I was out ridding with my son on his 2001 KX65, having fun on a wide open cane road. About an hour into our ride, my son was ridding in third or fourth gear wide open, doing 25+MPH when suddenly his bike let out a HUGE belch of white smoke, then a second or two later his rear tire locks up and he is sliding fishtailing to a stop (wish I had my camera, his eyes were as big as plates, his little heart was going faster then his bike, so scared he was studdering). Got him calmed down and took a look at the bike.
A little white smoke was still coming out of the pipe. Rolled the bike back and forth, back wheel rolled. Carefully kicked it over, the engine turned over. Took out the plug, I could feel compression and suction with each kick. Plug looked good, nice tan/gray color with a good spark. Used an allen wrench to confirm the piston was moving up and down properly. Had coolant, engine oil was good not contaminated with water. Engine didn't seem excessively hot.
Put everything back together, kicked it over and it started up, but sounded terrible. Had a deep boggy sound instead of its normal high pitch pop-ping. Thick white smoke out of tailpipe. Also had a definate clack/rattle on the right side. Shut it down immediatly.
It took 20 minutes of convincing my son we had to tow it back to the highway with him ridding it. He wouldn't let me get out of first gear, kept on dragging the brakes, suprised my bike didn't overheat. It towed fine while in neutral, got back to the truck and back home. By the time we got back home, did my boy have a wild story to tell mom (and did I get some dirty looks).
Where do I start looking? My guess was a siezed piston when the rear tire locked up. But the piston is moving freely. A blown ring?  How can I tell if it if the bottom end crankshaft and main bearing?
BTW this is pretty much stock KX65. I've only added boyseen power reed and a flywheel weight to tame this little beast and make more tractable low end. I have been using 32:1 MC1 oil with premium gas. Have always let the bike idle warm up for 5 minutes before ridding.

Any help appreciated.

Aloha,
Cory


Answer
Hi Cory,

That's quite a first experience for your son. Interesting story, too.

I agree that something seized. You'll have to perform a tear down to inspect suspected damages.

Look at it as quality time teaching your son about maintaining an repairing his MC. allow him to get involved with the work.

Perform a leak-down test to help determine the source of the leak in combustion chamber.

If you want more detailed intel, reply with an email address. I return tech ifnormation files by email. (AllExperts doesn't provide their experts with your email addy.)

It helps to buy the wifey a movie and expensive dinner.

Respectfully,
Mark Shively