2015 CRF450 Oil Loss

I did a top end on my bike and rode it about 1.5 hours with no noise or indication of anything wrong. It locked up on me out of the blue while riding and I couldn't move the kickstarter. I pulled the motor apart and the piston was all jacked up. I drained the oil and there was barely any in the bike. What possible reasons could have resulted in that oil loss? I had a small pinch in the O ring on the oil filter cover and I found a tad bit of oil on the bike below that but it didn't seem like it'd be enough to seize a motor. I would think I'd see oil all down the frame and on my boot and pegs if that much oil blew out of it.

Drain plug was in and I didn't find any holes in the case. Only thing I can think of is that I somehow installed the oil sweeper thing in wrong and I just burnt up blow by, but I would think that'd be pretty obvious due to the bike smoking a lot.

Any ideas on what else to look for?

Also, any help on how to determine what needs replaced in the bottom end would be helpful and much appreciated.