You can either remove the head and have the hole repaired at a shop or you can go get a helicoil insert and do it yourself. I'd say these are the 2 extremes. I used the helicoil insert on my daughters 86 xr 80 and its worked fine but I haven't changed the plug in over a year. I'll probably have to install a new insert when I change the plug. On a newer bike, I'd use a machine shop to have it done. Either way, its an easy fix.
The TT-R 125 uses a C7-HSA. That plug has an 10mm thread.
I cannot promise that it will work on a TT-R, but I have had good results with some Hondas; namely ATC70s and Z-50s, by simply tapping out the existing hole and using a 12 mm plug, a D8-HA. Same reach, same heat range, bigger threads.
The thing to look out for are valve seats.
Pull the head? Nah, not necessary. Unless you can't get to the hole because of the angle, frame interference, etc. When drilling and/or tapping; keep some sticky grease on the tool. Most if not all of the chips will stick to the tool. If a couple fall down in the cylinder, no big deal anyway; they are just aluminum.
Perhaps some chips will lodge under a valve, but I have done dozens of these, and have never had a real problem. They usually crank right up.
Be warned; do it correctly; if you screw it up; it will be a new head. $223.00. But then that penalty goes for installing an insert as well.