Hi,
If you were to upgrade to a 180mm rotor you would need a +20 IS to IS mount adapter. This spaces the caliper further from the fork leg allowing you to fit a 180 rotor.
Obviously your braking power will increase slightly with a larger rotor.
I had this as my first trials bike. The tektro front brake really lacks power and doesn't have very much lock.
My advice is to upgrde the brake now and save yourself money in the long run. As I'm sure everyone on here will agree the "Avid BB7" is an excellent choice. They are around £60 (160mm version) new and come with a rotor. Even if you only swap to the BB7, even the 160mm, you will be amazed at the power it has, and because its a cable brake it will work with your existing lever and cable for now.