GTO 125 is already a beast but to to ensure peak performnce there are some tips which i had used back in 1997 while owning a GTO 125.
1) Use 2 stroke oil through automatic lubrication system.
(The best two stroke oil vailable in Pakistan is Revtex 2T by Caltex. In other brand Shell advance 2T is also good but not compare able to Revtex.)
2) Good air filter should be used & replaced timely to ensure free flow of air intake.
3) Performance plug like split fire can be used.
4) Replace ignition coil with Genuine one if its not genuine already.
5) Ring/Piston kit. rotary sucktion plate & its cover block must be genuine should be Genuine