IN 8 YEARS I HAVE NEVER ONCE HAD A CATASTROPHIC MALFUNCTION WITH THIS BIKE. IT IS BUILT LIKE A TANK.
I replace the top end every 50 hours or so. Did I mention I race regularly in the MRANN Desert Series in the B class (Expert) with this bike? I have never had a race that I did not finish due to malfunction. I race in 100 plus heat and over 100 mile races, and never had it overheat even on the most grueling enduros.
If you maintain the bike properly, replace chains and sprockets, the occasional seal or gasket here and there, you will never have a problem with this bike. There is a reason KTM is the number one selling bike in the world and now in the U.S. They are RACE READY as KTM advertises.
The 2003 KTM 125 SX is a dream of a dirt bike. I have been riding since I was 6 years old - just turning 30 - and I have owned over 6 bikes, both 2 and 4 strokes from practically every make and model. I have bored this engine out to a 144cc and changed the gear ratio to fit it. I live in Northern Nevada in the high desert, and once you jet the bike correctly for your altitude and run the proper fuel ratio (40:1 for me... some run theirs at 50:1, but this is all relative to one's elevation jetting etc), you should never have any trouble - EVER - if you do, you unfortunately got the occasional lemon off a production line or it's your own fault. These bikes are very easy to maintain and really don't require much maintenance to perform well. Proper maintenance, as common sense would tell you, just makes it that much more reliable.
It is extremely fast for a 144cc, lots of low end, enough top-end at 70 mph and revs to the moon! It is light and nimble in tight sections, and smooth and forgiving in wide open and "rock-gardens" due to its ability to lug down low and rev back up at the drop of the clutch. There is a reason as well that most Pros race 2 strokes in off-road. Many of which they ride 150cc-300cc depending on the environment... They work. It will be hard for me to ever race a Japanese bike again off-road.