Yes you can use a Lipo or LiFe two cell pack with a voltage regulator. 5.0v is a fairly optimum voltage for them. The CDI ignition module draws something like 350ma at low speed and something like 650ma to 750ma at full speed. It varies depending on the CDI though. Older CDi's could draw more current too. The newer CDI's tend to be more frugal though.
Like the others you need at least a 2000mah capacity pack regardless of the battery type you use. The CDI can draw down the battery pack pretty fast depending on your flying style.