Warmer heat range spark plug, but dont move up the scale to fast from what your using now.
Dropping the needle can effect off idle response for sure. Those stock needles can look like a thumb instead of a index finger . The index finger having a more tapered look.
Hmmm finding a good jet and needle racing kit for the KTM 50 carbs
Its possible , i would call around to KTM and try talk with a guy out the back.
I had a similar issue with a 50cc scooter , carby is not sophisticated , the needle looked like a thumb. All i had to do was talk with a guy out the back and he presented a very awesome "racing needle and some jets",
15euro and problem solved.The front desk guy didnt know how to figure it out.