2003 KTM 250 SX won't start. When you kick it, it starts to fire but never turns over. I accidently put the wrong gas in it. It takes 60:1 but I didn't put enough oil in it. It died while I was riding it. I drained the gas and put new in it. Changed the spark plug. Any ideas on how to get it to run?

That's liquid-cooled? The problem sounds VERY much like water or coolant getting into a cylinder.

could be as simple as water in the fuel.
kick starter issue: need to remove side cover and inspect!

could be a faulty spark plug or the wrong one.

check the carb float level and mixture settings

Where is it leaking from? If its leaking, that's more than likely why it doesn't run right, the fuel/air mixture is off. Also, I wouldn't continue to run it until the leak is fixed as it may damage the motor from not lubricating properly.

check the barrel for any even wear.

Why 50:1?
All bikes should be 32:1, and then jet to match that.
Your oil is fine, and yes...go smaller on the pilot.

Black Plugs are indicating that your bike is running too rich(too much fuel to air mixture) you need to have the carburator adjusted,more air.Good Luck.

Correct me if I'm wrong but the 250SX is a 2 Stroke engine with a Gas (1.98 Gals Unleaded Premium 91 octane Recommended) to Oil Mixture (using Motorex Cross Power 2T at a 40:1 - 60:1 depends on oil quality) and when KTM says to Change the transmission oil the amount called for is 0.7 liters engine oil using Motorex Top Speed 4T 15W50. These Quanities are from an 04 250SX could not find a 2000. So consult with your local KTM Dealer's Service Department and pickup a Owners Manual at the same time it would be in your best interest. Liter breakdown 0.7 liters = 0.739681 quart (US liquid) OR 0 quart (US liquid) and 1.47936 pt (US Liquid) / 0.7 liters = 1.47936 pt (US Liquid) OR 1 pt (US Liquid) and 0.958726 cups (US)