Solo 2350D won't start

Nadler could be on to something here, intermittent coil failure. Especially if this is a know past fault point.
If you did a fuel flow test and rebuilt the carbs, It points to a intermittent electrical problem.
You need spark, fuel and compression.
What makes you certain the over rev cut out is not kicking in?
Can you disconnect it for testing purposes?
If the problem is intermittent I think the Plugs might be OK.
If not dropped plugs are pretty tough, simple and trouble free
but you never know.