Fisher Price 12 volt power wheels

like these ones here
http://www.modifiedpowerwheels.com/

my son wants to modify my grand daughters pick up truck power wheel

these things are powered by two motors that look like emaxx size motors..they use a 12volt battery...some of the guys on that site series two and three batts together

we are wondering if you could use lipos like we use or nimhs batts to power these things?