Sorry to be the bearer of bad news, but you will be rebuilding it. I am surprised it even survived in running condition, I guess that's a testimonial to the reliability of the KDX. The knock is certainly the rod bearing, and you will probably find a lot of scoring on the cylinder and piston. The refusal to idle was the extreme friction, and probably the beginning of a complete seizure. You are probably looking at $300-400 to rebuild the crank, replate the cylinder, replace the crank bearings, and replace the piston and ring, assuming you dissassemble it yourself. Add another $200-300 if you have a shop do it all.