I had a 00' 125exc and thought it lacked too much bottom for me.
It would be easier and cheaper to sell it and buy a 200.
The carb is the same with slightly different jetting but you would need a pipe, cylinder, head, piston, probably a conn. rod, and maybe a crank.
With KTM prices this could add up to a bunch of $.
A 144cc big bore kit would be a option.