A lot will come down to cost, and how lucky/patient you are in finding good deals.
I would take a look here
http://www.evalbum.com/ , click on the search tab and go to 'motorcycles'. Then just look at what everyone else is achieving and what you can realistically expect.
Lets say the bike uses 120Wh/mile at 60mph. The classic Austin Mini car uses 10HP/7.5kW at 60mph, figure on a bike using less than that. How much less is down to aerodynamics and rolling resistance. Say you achieve 6kW power requirement. (I suspect that is too high a figure, but lets see, for example, how you might approach specifying your components)
You need to break down that 6kW into battery power.
Because P=IV (Watts = Amps x Volts), lets say you have a 60V pack, then Amps = 6000/60 = 100A
This means you need a battery pack capable of delivering a sustained 100 Amps. for good acceleration, you can expect 350Amps, or more.
So start looking for batteries which will support a 100Amp discharge rate, and allow short duration pulses of 350 or more Amps.
So what about the motor ? Typically, on a single speed bike, gearing is set to about 5:1 via the sprocket ratios.
To get 60mph from a tyre on an 18" wheel with 5:1 gearing, expect a motor rpm in the area of 4000rpm.
So you need to be looking at a motor which can give you 6kW (maybe less) at 4000rpm.
there are some other things to be considered too. Because P=IV, you can see that for any value of P(in watts) that as V increases, I decreases, in other words more Volts, fewer Amps.
This is important when it comes to wiring. Higher voltage wiring doesn't need to carry quite so much current, so the wiring doesn't need to be as heavy gauge at high voltages as it does for low voltages.
Using 60V/300Amp as the example, while your cables ought to be able to withstand 300Amps for at least short bursts, if yiou rearrange the battery to deliver 120V then the cable only needs to be good for 150Amps (assuming the current will be limited by the controller or some other device - fuse)
Another important aspect to consider is the power losses in the cables due to cable resistance. Every cable has a resitance, and the power lost in any cable is proportional to the resistance times the current squared, Ploss = R x I^2. Easy to see from this that keeping the current down and voltage up high that yoiur connectino and cabling losses can be reduced significantly (never eliminated completely though)
