The battery that came with it is probably a 7.2Volt 900mAh NiMH. You will need to buy a new charger if you go Lipo.
The speed and torque will be slightly greater than the NiMH becuase the voltage goes up from 7.2v to 7.4v.
The run time will be totally dependant on how many Amp hours the lipo battery you choose is. if you could get a 900mAh Lipo at 7.4 Volt it would last fractionally more (say 3 minutes).
Lipo's are smaller for the same Amp hours.
On my AMP the biggest NiMH I could fit without modifying is 3600A/Hr With lipo I could go to 6000A/Hr without mofifying, so run time would be about 1.8 times a NiMH.
Without knowing specifics about the battery you choose, I would say almost double run time if you go as big as will fit the present battery housing.