I don't have time to re-read all of these posts so let me try to summarize from memory and you can correct me.
You bought the ped, it had not been run for years but ran fine.
It began "4 cycling" (my terminolgy) at about 25 mph.
Removing the air cleaner and box did not help.
Turning off the gas at full speed caused the power to return just before it died.
Cleaning the carb did not help.
Decarbonizing the cylinder and exhaust port did not help.
Your ped is running too rich.
Maybe the carb was partially plugged when you got it and running fresh gas through it cleaned it up some and you need a smaller main jet.
Maybe your needle and/or jet are worn.
Maybe there is fuel in the float, affecting your fuel level.
I am surprised that removing the air cleaner and air box does not help but I guess yours must be a really good design.
Review all of this then re-post.