Could very well be a carb and/or valve problem (perhaps electrical?).
Putting the engine apart again will be your last step if all else fails.
Incorrect valve clearance, dirty carb, lack of oxygen or a mineralized
spark pug (or weak spark) would all cause your engine to greatly
underperform; how's your battery? Lose coil perhaps?
Low RPMs (specially 2dn gear) are incredibly powerful in this model.
Try simple stuff first just too eliminate variables.
- recharge battery
- metal brush your (+)(-) terminals
- clean/change the plug
- test the spark itself
- richen the mix and try a hot plug maybe
(screw counterclockwise about 2turns out)
- do a test run with the airbox door off, vacuum you airfilter
- redo the valve adjustment (must not remove engine at all)
making sure of your TDC (use a wire or your cars' dipstick
in your plug hole as guide). Your exhaust valves could be too open.
Good luck