2005 ktm 450 sx die while riding.

I'd start with simple things like checking the plug and doing a few electrical tests.Some tests you may be able to do yourself like resistance checks.Compare the following spec with what you get ; Primary coil . 25~35 0hms Secondary 5050~7550 Valve clearance inlet 0.10~0.15 exuast 0.15~0.20 both cold and in mm.

http://www.ktmpartsonline.com/manuals/download.php?f=2005_450EXC.pdf

Check the post you submitted yesterday. There are a couple solutions listed for this issue.

Will it turn over at all?

If battery is running down. The engine may not be charging the battery. Check charging system. If running on battery, the bike will run fine for awhile then die as the battery can no longer provide spark to the engine. If charging system is less than 13.0 volts could be a ad stator or regulator. Make sure the battery posts are clean, a bad ground can cause problems too.

The decompression valve has come apart in the engine. Tear it down!

I also had a bike rebuilt on the top end, which also poped when kick starting. I thought it was going to start, but never did. I used a compression tester gauge on it and found that I had 60psi instead of 100psi. I had it rebored buy a different shop and they informed me that the last shops equipment bored the cylinder oblong instead of cylindrical.
I was out the money from the first shop but I did get it fixed. Hope this helps

Hi and welcome to FixYa,

Offhand, the only factor to be checked that could foul a sparkplug for your bike would be the 2T oil injector/pump. There is too much 2T oil going to the combustion chamber. Either the injector would require restricting or for some reason, there is also 2T oil in the fuel tank (pre-mix).

Good luck and Thank you for using FixYa. Happy Holidays.