The problem with putting a big bore kit on that engine is the clutch won't cope. The clutch on pretty much all chinese engines up to 125cc is on the end of the crank, called a primary clutch(ie clutch spins as fast as the engine,one revolution of the crankshaft = 1 revolution of the clutch). For anything bigger/more powerful the clutch is gear driven off the crank at a reduced ratio, a secondry clutch (ie the clutch only spins at maybe 1/4 of the speed of the crank). The reason for this is there is only so much power you can put through a primary clutch before it goes boom (and possibly fly through the cover and take your foot out at the same time).
TB used to do a 146cc bore kit for 125cc motors but by the time you buy the kit and work out how to stop it killing clutches you could have bought another motor that will easily outlast your built up motor. Your cheapest option is to buy a s/h pitty with a bigger motor, swap out motors between the two and recoup at least half selling new bike with the 125 in it. Dont be fooled different looking clutch case or by the terra moto on the clutch adjustment cover, that engine is just a run of the mill 125 from over 10 years ago, i have the same style 125 motor in an old pit pro i bought for $100 a few years back.
If you are looking for a bore kit measure the head/barrel stud spacing, those motors run the smaller spacing (like a Honda) where most of the newer 125s run wider spacing
If you're keen to get a bit more out of that engine, a flat slide carb will make if feel a lil faster, and i am pretty sure you can fit a z40 cam if you take the bearings off the z40 and use the ones on the cam currently in your motor
very different looking big bore cylinders.
