More than likely you will need to reinforce your Power Wheels car frame to support the weight of batteries, motor/motors, and a grown adult driver. Or just scrap it completely and start over.
Lot of imaginative options are out there, but most common is to weld up a steel frame. Some people use old bed frames, some use square tube…
You can build a frame, or part of it, using wood. This has its own pros and cons…
Some teams have used 8020, and at least one team was investigating the use of Unistrut.
Useful conversations pilfered from Facebook posts