Heya!
As far as batteries go, from what I know the higher the voltage, the higher the top speed. So you might be alright with 48v, if you're only going down back streets. But if you plan on going down a freeway you'll struggle.
I suggest you look up on youtube for some EV bikes. Don't be distracted by the killacycle, even if it is a worthwhile bike to emulate hehehe. There are a couple of bikes out there that are quite good, and don't look all boxy and ugly with batteries hanging out in the wind. Remember that with bikes, aerodynamics are quite important - since there isn't much room for batteries.
Also: I suggest you fiddle around with the <a href="http://www.evconvert.com/tools/evcalc/">EV Calculator</a> for a bit. It's not 100% accurate, I suspect, but it'll help you get the gist of what you'll need. Though I don't think it includes "motorcycle" in it's presets, you you might have to try and find some stats for your bike.
I was going to do a bike myself, but after some thought I realised it wasn't very practical for me. I'd need to carry my toolbox at the very least (so another 10kg or so), and to be really cost effective I'd need to use it as often as possible - so I'd need to be able to take passangers.
But that's just me. There need to be more EV bikes out there! hehe
*edit*
Oh, silly me, there's also the EV Album ( http://www.austinev.org/evalbum/ ), which has quite a few bikes in various states of conversion.