Had a quick look online. Ybr125 doesn't look like it has a removable sump pan which leaves two options, split the engine casings or helicoil it in situe.
I'd have a bash at doing it in situe as stripping the engine is a big job.
You'll need the correct size drill bit, make sure it's sharp (don't be tempted to use the nearest size you have available, buy the size the helicoil kit recommends). Drill the damaged thread out very slowly then use cotton buds and tweezers to remove all the swarf, a cheap endoscope will also be your friend here (you may also be able to remove the clutch cover and clutch basket to get access to the inside of the sump plug hole). Use the supplied tap to run a new thread through the hole you've drilled, again, tweezers and cotton buds to remove all the swarf. Finally screw in the helicoil and snap off the metal tab, make sure you extract the metal tab afterwards. Job done, panic over.