GTO 100/110 can be modified with over sized piston of GTO 125 but in case of GTO 125 Itself i dont Think u can do major modifictions. But u can do optimization of ur stock ride for peak performance.
I had own this Bike for three years & how i do maintain my bike is as below.
1) Piston/Rings set should be genuine instead of Japanease said in market.
2) Cylinder also needs to be Genuine doesnt matter what the size is but sleeve shouldnt be rapaired one . (U can also use Indian & Irani Cylinder coz they r also Genuine but if found from relaible person).
3) Genuine CDI Coils + High performance spark leads along with atleast Denso Spark Plug or u can have Bosh or split fire plugs too if available with same specs.
4) Use twostroke Engine Oil in side oil bottle/Pump instead of mixing oil in Fuel tank for lubrication. Revtex 2T by Caltex is the best two stroke oil available in Pakistan use that one.
5) dont try to Use Hi octane fuel just Use Standard Fuel.
6) Clutch/Presure plates, Carburator, Sproket /Chains shouldbe well maintained & aligned.
7) Rear tyre profile shoulbe not more than 3.00-18 or 100/80 -18 incase of tube less tyre.
Dont try to use over sized drive sprokets coz this machine is alredy under touqued so it will die in fifth gear if u use them.
Just follow these steps & u will feel the diffrence in over all performance i.e. Pick up, speed , & fuel effciency.