Only 4 really important things to look at when selecting the battery
1) Voltage, in this case you'll want to stay with 2's or 7.4v
2) SIZE, in this case VERY important... as long as your new battery is the same length,width and thickness you're golden... IF you can fit a slightly larger battery the following is also important...
3) Capacity, this not only dictates how long your flights are it also can increase or decrease the weight of the battery to the point CG is shifted... in most heli's you don't have a lot of wiggle room and if power is marginal as is adding weight is the last thing you want so keep the capacity right around 450 to 550Mah.
4) Connector, not all that important really as most of use change them anyway... HOWEVER if you don't want to fool with it then buy the right connector all ready attached

in your case you need a JST connector