Your only real option is to use some large rc batteries
6 or 9 of these would do well
then wire up a bms using rc conectorshttp://www.hobbyking.co.uk/hobbyking/store/__34740__Turnigy_nano_tech_5000mah_10S_35_70C_Lipo_Pack_UK_Warehouse_.html
other than that its a headway job.
(that can get expensive real fast)
I spent nearly 3,000 on a 120ah 48v battery
(worth every penny)
speed calculation is very easy if you know the rpm
mesure round the wheel in meters
then times it by the rpm.
This will give you the speed per min so times it by 60 and your done.
(the anser will be in meters per hour so divide it by 1000 to give you KPH)
im guessing your wheel cercumferance is 1.57m
1.57 times 620 =973
973 times 60 = 58404 meters per hour
58404 div 1000 = 58.4 kph
36.2 mph not bad