Ditto the above, from my experience.
Quickest answer is to buy and install a 56 tooth sprocket.
The 66cc (not really 80cc) motor will go up the big hills then.
I'm running a 48cc with 56t and it will go up the hills ok.
Some pedaling might be needed to keep speed up, but little effort.
I stuck with the stock 44t sprocket on my 66cc and opted for motor work to get the power to get up hills.
Read my old posts, but basically sanded the head to get some squish and compression, matched the exhaust gasket and pipe flange and leaned the engine fuel mixture slightly.