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.