Wow, year old thread so it looks like I'm late. But for anyone else wondering, the vehicle doesn't care what battery chemistry is used. As seedy stated, as long as it's the ESC knows it's LiPo vs NiMh, you're good and that 0.2v nominal voltage difference won't matter. For a 2S battery (2 cells connected in series), the C rating will most likely be very low or just wrong on the label. So just charge it at 1C, meaning charge the 4200mah battery at no more than 4.2amps and you'll be good. I'm more curious if the battery would physically fit. I suppose it's probably the same size.