I'd recommend either the 21T (00968-2939) or 23T (00968-2952) boxes. The 23T comes out of the Corvette and should drop in just fine. As an additional bonus, it should come with a slightly faster (~20k RPM vs. 16k stock) motor -- at least the one I got did.
http://www.mltoys.com/Power-Wheels-Corv ... 8-2952.htm
ebay listing for the same: https://www.ebay.com/itm/Power-Wheels-G ... 5762!US!-1
You'd need to replace both gearboxes. I don't know that I could guarantee you a source for a single gearbox that would be the same speed as the current ones.