@ 12V the original Speed 900 draws 54A max. Let us take that as a given. (I messured more, but hey, that's another matter)

54A for 3 minutes is 54/20= 2.7A used. LiPo doesn't like to be deep-discharged,so, we add 25% to that= 3.375A

A 3S 3500mAh 20C pack will do. Theoreticaly.