1. Additional gearbox and motors with correct pinion gears.
Answer: I still need to open up the gearbox and see what I'm working with.
Chart: Depending how involved you're wanting it does look like MLToys may have the first gear if you choose to replace it. At 18v you may be ok but just know they probably won't last forever. If you do go with steel first gears you'll definitely want to verify they're the correct one by comparing them with your own gearbox.
2. May need a longer axle to accommodate the extra gearbox
Answer: It appears there is space to add an additional gearbox/motor to the axle as well as room and a cutout inside the battery compartment. I do require an adaptor to add onto the wheel assembly which I found here.
Chart: If you do go with the one from the link make sure the sleeve measurements are the same as yours. The link you provided shows several different hubs and one of them may work with some modification but I do have concerns. My worry is how the rotational force is distributed throughout the wheel hub. If you notice, the original wheel hub sits inside of a cavity or depression on the wheel that's the exact same shape as it is. This helps to better distribute the forces instead of focusing them at the screw points only. Think of how a wrench works, more surface area in the direction of the rotational force provides for better force distribution - less likely to strip the bolt. If it were metal I wouldn't have any concerns but since it's plastic I worry. Needless to say it's something to think about. I tried searching the web for the exact wheel hub but no luck so I ended up contacting Huffy. They provided me with the parts and price information below. It looks like the wheel hub can't be purchased separately but at $13 for the drive wheel you'd get a new/spare tire out of it (or a tire to experiment with) that comes with the hub already mounted to it. If interested just contact Huffy support at email@example.com
Drive Wheel ($12.99) PN: 17327-9
Drive Motor ($19.99) PN: 17327-10
Motor Gear Box ($12.99) PN:17327GRBOX
3. Need a new wheel hub for the wheel without the ability to connect up to a gearbox (if the rear right wheel is different in terms of mounting the hub than the rear left you may need a new wheel also)
Answer: I believe the adaptor listed above should work with the existing wheel with a little modification.
chart: See my comment above.
4. Determine how you plan to wire the additional motor, series or parallel or both? If both then you'll need an additional DPDT switch
Answer: Not quite sure yet
Chart: If I remember correctly this particular Lightning McQueen doesn't have a low and a high speed so the shifter is only going to provide reverse or forwards. Now you can, with some modifications, add a low/high speed feature if you want. It wouldn't be integrated into the shifter but you can add it as a "boost" button on the dash which would be pretty cool. If you choose not to add a low/high feature, as mentioned before, just stick with a parallel configuration - equivalent to high speed all the time. If you need help on the wiring just let me know and I'll get/make you some diagrams.
5. May need to replace the switch in the gear shifter with one designed for higher voltage/amps
Answer: Sounds like a smart idea
Chart: Most shifters have a DPDT rocker switch inside. When the shifter handle is moved an arm inside the shifter slides up and down the rocker switching it. You've got two options here. You can either find a high current DPDT rocker switch that can fit in the same location as the current one with minimal modification to the switch mount (this isn't easy) or just get a high current relay and use it in conjunction with the existing switch. The only thing the relay does is allow for a small switch to be used in a high current application. High current automotive relays are about $10 give or take on Amazon. Again, if you decide to do this and need help with the wiring just let me know.
6. May need to beef up the wiring
Answer: Yes, the current wiring is 18 gauge
Chart: 12 gauge would be ideal. I actually just purchase 12/3 extension cords on amazon which work perfectly and it's cheaper than actual power wire if you go with Amazon's in house brand. With the extension cord options you may need to actually cut the wire out of the exterior sleeve, especially if you want to route the wire in the same place the existing wire is, which is what I do but in my opinion it's not a big deal
7. Is there space to house the necessary batteries for 18v operation?
Answer: Yes, plenty of room under and behind the seat.
8. Are the existing lights/electronics going to accept 18v (headlights, lightning bolts, etc)?
Answer: there are some extra lights as well as a speaker so I will need to modify those to a stand alone 6v battery.
chart: The speakers are typically ran off of AA or C batteries so I think you're fine there. The lights and/or sound boxes if there are any may be a different situation so you would need to either run them on a different battery or add a power divider. Id' venture to say the lights would be fine with 12v but 18v or more probably not
High current relay:https://www.amazon.com/dp/B012N09BFE/ref=sspa_dk_detail_4?psc=1&pd_rd_i=B012N09BFE&pd_rd_wg=YdXTj&pd_rd_r=N9B5NZ9ZT5TX6896M5PY&pd_rd_w=YTuD1
50' 12/3 HD Extension Core:https://www.amazon.com/TES-SJTW-Heavy-Extension-Lighted/dp/B019GT5G4U/ref=sr_1_16?ie=UTF8&qid=1509732425&sr=8-16&keywords=12%2F3+extension+cord