You've already upgraded to 24V -- are you using an ESC?
Mltoys has 7R gearboxes for Escalades here, though I don't know if they've changed it over the years. http://www.mltoys.com/Power-Wheels-Brute-Force-Gearbox-p/00968-2819.htm
(This is according to their gearbox reference list here: http://www.mltoys.com/Power-Wheels-Gearbox-Identification-Guide-s/130.htm
You'd then need two 775 motors and the appropriate pinions. Cheapest 775 motor is the Jameco ones ($16 shipped for 4):http://www.ebay.com/itm/Nichibo-Taiwan-775-7013F-R-12VDC-Motor-12-610-RPM-4-pcs-/112230651034?epid=503587959&hash=item1a2177d49a:g:i5gAAOSwImRYSaDp
These are good for 33% more torque than stock and you get the larger can size (better heat dissipation, etc.). They can be run with stock wiring. You can spend more and get the Banebots (Mabuchi 8514) for ~$45 for a pair of them but you should invest in an ESC or other motor controller if you're going to run those.
Pinions are another $8-10/ea, so you're looking at a total of $120-$130, depending on shipping. Even if you bought the 775-conversion centering shims from mltoys you're only in $150. If you go with the cheap motors you knock $30 off of that.
But if you're going the 775 route and want more speed, you don't buy the Brute Force/Escalade gearboxes. Get the Corvette gearbox instead (http://www.mltoys.com/Power-Wheels-Mustang-and-C7-Corvette-Gearbox-p/00968-2821.htm
), buy a set of the cheap Jameco 775 motors, and two 22T pinions. This should be a 30-35% increase in speed with no torque loss and no additional wiring stress vs. the stock system (motor has more torque, the different gearbox ratio turns that torque into more speed).
F-150 - 24-36V with homebrew 100A variable-speed motor controller, 4x 775 motors (no, it's NOT all-wheel-drive--look HERE
for more details!)
Pink Rocket Princess Mustang - 15mph, 36V, 4x 775 motors, homebrew PWM controller rev 2....
Hurricane - 24V w/50A step-down controller, 775 motors