Airbox is fine. No Leaks and normal restriction.
Exhaust is clean as well as the port.
Tranny fluid is at it’s correct level.
No air leaks anywhere.
New spark plug, and spark is quite strong."
With all the above meeting factory specs, the only things left that are suspect are;
Do you have the correct carb?
Do you have the correct jet?
Is clip setting for the jet needle in the correct setting (second notch from the top)
Are the clutch shoes slipping?
The correct carb and jet numbers for a Puch MK II should be;
Carb Bing #1/12/318 with jet #50 for a 1.0 hp model 20mph
Carb Bing #1/12/323 with jet #48 for a 1.5 hp model 25mph
Carb Bing #1/12/322 with jet #60 for a 2.0 hp model 30mph
If you have the correct specs above I would suspect worn or slipping clutch shoes.
Let us know what you have as far as carb and jet. Carb numbers are on the outside of the carb, jet # stamped on head of main jet screw.