OK, here's the real trick here. I did this all the time with my daughters barbie Jeep and she loved it. Take an 18v DeWalt battery and use aligator clips to hook it where the standard 12v battery goes.
*Prolonged exposure to 18v will damage motors*
Best thing to do is find donor motors out of other power wheel vehicles. There is usually a find at the yard sales and dump at the right time. Have a blast and do donuts on the pavement!!