Yes the casings are waterproof. Most conversions I've seen are in cars, but the rough estimation here is that by the time you rip out the gas tank, transmission, and swap engines and replace the empty space with cells, you're going to get roughly the same per tank range as a gas counterpart. That means a car goes roughly 200-300 miles a charge and a bike gets about 75 under normal driving conditions. As FG mentioned, the cool thing is the faster you suck the current out of the battery, the more torque you can get. Under racing conditions, I have no idea what you could get. Then again, if you're racing, I'm assuming you're bringing your bike on a trailer so you can also bring tools and whatnot. You could easily drag along a few extra cells in that case to swap out between heats.
The cells are as big or as small as you want them. Most Li-Ion cells are made up of several smaller cells like this http://www.lsbu.ac.uk/solarcar/solion.pdf wired up in series, then sealed in a waterproof container. Its all very custom. Could you design the system to be able to take extra cells with you? I'm sure. None of it is very off the shelf at this point, so its really up to you and your imagination.
I will leave you with this as inspiration : http://www.youtube.com/watch?v=8qDZOBQs60w
have you seen any of the electric drag racing cars, those things are crazy fast! i think my bike goes about 200 miles on a tank. it holds 5 gallons of gas. and it 600cc so .6 litre.
i can cruise at about 3-4000 rpm. in 6th gear (but 17,500 is much better. lol)