Oil spewing from breather hose

[TxState]

I have a 125cc chinese dirt bike that "blew up" several months ago and has just been sitting. I chalked it up to being a chinese product and never bothered to try and diagnositics. Recently I've been looking at buying a new motor with this coronavirus check. I wanted to understand what caused the failure so that I don't blow up the new motor I am about to buy.
I saw this post (https://www.miniriders.com/threads/gpx-125cc-problems.42298/#post-485410) which basically described my symptoms to a T:

"With your GPX did it hole the cases and leak or just blow a lot of oil out of the breather near the back engine mount? It sounds like you were revving the crap out of it for a prolonged period of time, if so its not that unusual to blow oil out of the breather. They are not really designed to be held wide open constantly for prolonged periods."​

This is basically exactly what happened. I was driving the bike to campus (first time driving it this distance) and had been riding the bike at wide open throttle for about 20 minutes straight. I noticed it losing power suddenly, then the bike died. I pulled over and noticed that the motor had spewed loads of oil out the breather hose on the top/rear of the motor. I tried to restart the bike but didn't have any luck.
Can someone explain why this motor failed? I'm reluctant to replace the motor without knowing what failed and why. Can air cooled 125's just not handle any significant distance? Is it an RPM issue? A heat issue? The bike wasn't very old when it failed. Do these 125 motors have limitations that I wasn't aware of?
Thanks for any insight you can offer.