With my sons Power wheels Jeep in storage for the winter, I thought I would at least provide some info and link. modifiedpowerwheels.com is a great site with tons of info about modifying these battery powered chariots. They have a great forum and a ton of info.Be sure to check out their videos page as it cover the majority of what you might want to do.
The above modified Mustang has a Aftershock motor and 18volts of power. check it out here. www.modifiedpowerwheels.com/forum/topic.asp?TOPIC_ID=20427
If you are looking to get all your info in one space…. You can check out http://powerwheelscd.com/. I have their CD and the info they provide is great and very helpful.