[Bob KellyIII]

it's very unlikely ,but possible that you've lost a tooth on the crank end of the timing chain....
You have a set of extremely ODD symptoms with that machine !
1. Not reving high almost means not enough fuel being delivered.
2. spitting back through the carburetor can mean, too much gas or valve timing.
3. it ran good and while running started running poorly. (failure in action)
...
You've already tried the spark possibility , no change.
You've eliminated the carb possibility, no change
You've got good compression...
through the process of elimination
that only leaves mechanical parts.....
the cams look good and there really in no good way to check for sticky valves
.... to me that points to the cam chain and it's components.
Yes a over head cam can jump out a tooth run poor and jump back and run great.... a super confusing issue but I have seen it happen.
....
after the tear down and re assembly and subsequent testing , are you SURE
you are using the right timing mark on the fly wheel ? and the cams with their marks.... they can't be close they have to be exact ! you can't hurry through that bit and get it right. ( well, I can't LOL)
keep working through using the process of elimination.... that process has never failed me yet in all my years !
.....
Bob.......