The Dune bug uses the fairly strong #7 Gearboxes, so you can put high speed motors in it and basically hop it up with 18 volts, nitrous button, etc.
The best motor swap for cheap is the all electronics DCM-231
You can't beat the price and they will run about 30% faster than stock. With either the high speed motor mod or 18 volts, the vehicle will go about 7 mph vs. 4.5 to 5 mph stock.
You can also swap out the gearboxes/motors with 21T pinion boxes from a PW Harley for about 20% more speed with no other mods.
I would suspect with nitrous and high speed motors you are looking at 8-9 mph. With a gearbox swap, Nitrous and high speed motors around 12 mph, but low end acceleration may suffer and it will take a lot of amps.
Go to 24 volts and you will see 13+ mph for 30 minutes to 30 seconds before something breaks or melts.
I would suspect with nitrous and high speed motors you are looking at 8-9 mph. With a gearbox swap, Nitrous and high speed motors around 12 mph, but low end acceleration may suffer and it will take a lot of amps.
I have a rubber tire mod on the CD. I actually had to re do it during development to slip a little, because the car would pull 2 foot wheel stands and my son got out, ran into the house and told his mom he was never driving Power Wheels again.
Most of Wil's videos are made with high speed motors and a nitrous button. parts for this would probably run about $40 shipped from the vendors on the CD. If you have a big driveway or a quiet street, these mods will provide lots of entertainment with very little part breakage.
If driving mostly on hills, grass or dirt stuff will likely get burned up (usually the $4 motors) with both the high speed motors and nitrous, so you should pick one to start.
Depending on the mod, the CD goes step by step to and wire by wire.
For a dune bug most of the wrangler mods would work. I don't have specific mods for this vehicle on the CD, like I have for the Wrangler, Mustang GT, Gator, etc, but almost all Power Wheels are wired the same, so you can easily adapt mods detailed for one vehicle to another.