"The term “kathoey,” which is often used to refer to transgender women in Thailand, has been used for centuries. Additionally, traditional Thai culture has a concept of a “third gender” known as “sak-sra,” which includes people who identify as transgender, as well as gay and bisexual people."
You won't be the only trans-person in Thailand, that's for sure. If you look like a male, and you want people to perceive you as a male, then just don't tell them otherwise, unless you want to. You will be interacting with strangers just the same as you do at home, so it's up to you to disclose or not. In my opinion, 99% of people will not care, and will leave you be like anyone else.
The only question I would have is regarding fighting rules, and if trans individuals are allowed to compete against formerly opposing, but now same genders. I don't have any knowledge about this. Researching 'Ladyboy Muay Thai Fighters' might be a starting point.
The only other thing I can comment on from your post is regarding scars; everyone in every gym has scars... wear yours proudly. And if someone asks, joke with them and say you took a double elbow combo to your breasts in a heated round-4 exchange with a former Lumpinee Champion.
Good luck