I'm sure the experts here will post soon but here are a few thoughts.
60 mph seems high for a 50cc. Mine does 50 but I weight 55lbs more than you,
so maybe. Lets figure you are doing over 50mph on the speedo.
What RPMs are you doing at this speed ?
I would first see what weight rollers you have.
They are too heavy probably, given your lack of take off
but have relatively high top speed.
Also if you have a tecnigas RS70 or RSII pipe, it's designed
for 70cc and by default you will lose some low end grunt, especially
running it on a stock jug.
If you are going up the south face of the Matterhorn it will slow right down but
a regular small hill shouldn't be a real issue. See if the RPMs drop or the speed or both and let us know.
That way we can see how you transmission is handling the load.
You can regain you bottom end by putting the stock pipe back on or change the clutch springs,
contra springs, etc and tuning the transmission to your pipe.
