Everybody has different opinions on the subject of when to replace the piston, and even more opinions on which type to use (stock vs. aftermarket). But as far as when to replace, I started noticing some hard starting at about 130 hours on the stock piston and couldn't trace the problem to anything in particular (I had replaced the original rings at about 80 hours). I put up with it for another 25 hours before replacing the piston with a Wiesco unit. After that, no problem with starting. I couldn't really tell if the piston was past its wear tolerance, because I didn't know what the tolerance level was to begin with.
Riding style will have an affect on piston life. Those who like to rev to the moon will probably replace sooner than those like me who do more lugging (that's why we have 300's right?).
I would say that if you haven't already replaced the rings, then you might want to think about doing at least that. With the top end apart, you can then measure the piston and determine if it's worn past it's tolerance.
I would be careful about the KTM pistons -they wear so well they tend to break up before they loose compression or go out of spec.This isnt a personal dig but too many KTM riders race/ride for years on end with the same top end and risk serious damage for the sake of $150.Vertex make good pistons that come on the 01 125sx as std, and you can pick the kit up for way less than from a KTM dealer.In the UK a 125 piston kit costs £60-why risk damage to the barrel for such a small amount of money.While the top ends apart check the powervalves etc for wear.Dont take this personally its meant to help not crititise.