Del,
This may be a longshot, but I have a friend who went through everything on his '99 Kx5 much the same as you are.
Jetting,float height, spark,kill switch, fuel. and the dern thing would just pop,pop pop...
He Bought all new ignition parts on e-bay and when he went to replace them he noticed the
woodruff key on the flywheel broke and every once in a while the bike would fire close enough to pop but wouldn't run.
Like I said it may be a longshot.
A fellow in the 250 threads had a similar problem with his flywheel/magneto going out. He ended up re-building the whole engine, the only thing left to replace was the flywheel. Replaced it and she lit right up.
I'm not for sure this will work, however it may relieve some of the pressure 'till you get the starting issue figured out.
A few of the serious engine guys here came up with a modified bolt for the KIPS Valve.
Long story short, I'm thinking if you unscrew the KIPS bolt and put a few washers on it it should allow your exhaust valve to over-rotate and "Leak" some compression out the exhaust valve.
I know what it's like to have 2stroke 500 foot. I ruined a brand new pair of a-star 6's in two days when I got my CR5. I probably could have destroyed it in one, but I was too out of shape.
Best of Luck,
Tuck\o/ EDIT BDI posted above as I was trying to send my reply..... maybe there's something to it