Re: What mod should I get.
The reason it runs better uphill is because your carb is jetted too rich. A smaller jet and proper tuning will improve power almost as much as a good tuned pipe.
My preferred mod list:
#1 : Proper jetting
#2 : Tuned pipe
#3 : smaller rear sprocket (I run a 36)
#4 : High-comp head (Puch, Fredhead etc)
#5 : Lightning CDI (No real power gain, just much better running motor, smoother and pulls from idle)
If more is better, then too much is just enough.
If you can't afford it, build it yourself.