You are going fast enough that you don't get to skimp on the Frontal Area and Coefficient of Drag estimations.

You have to start somewhere, so let me suggest

Figure out the CDa (and rolling resistance), you can determine how much horsepower needed to maintain 80mph with that.

That will give you tell you roughly how many watts you need (746 watts per hp)

Pick a motor that is a good %20 over that rating. Note RPM at that rating.

Figure out the motor power requirements at 45mph (in volts and amps, depends on gearing too)

pick batteries that can deliver the amps for top speed and can supply amps for an hour @45mph without losing more than half their capacity.

Note where you are at with the weight now (including driver) that you have a first guess on batteries/motor/bike/controller

determine your zero to 60 times at that weight/CDA/power level. If it is acceptable call it good, otherwise you need more motor/batteries.

Pick gearing (based on max motor rpm and tire size) to get you to 80mph.

See if that gearing is sufficient to get you up a 26* grade hill. If not you need more motor/batteries, or less weight (fancier batteries), or a lower gear that needs computing.

No promises, never been here before, just thinking out loud