I have been looking in to this and i really think that it is a serious product idea.
I found a website that has decent motors at good prices.
http://www.strongsvillehobby.com/enginepipes.html
These are not the cheapest motors there are, i remember last night i saw one for $50 but i forgot the link but i bet that if you find a wholesale provider you can get them for dirt cheap.
These motors come complete with the engine, engine starter, and all of the ports.
All you have to supply is the fuel tank and exhaust pipe.
The only problem i have is that you would need an adapter to get the vex square shaft to work with the motor or you would have to design your own motor to have the square shaft in it.
These motors have a crap load of toque, so much that they would shatter the teeth on the plastic gears meaning that vex would need to make some special metal gears and stronger shafts just for the motor kit.
Once you have the motor, fuel tank and exhaust pipe you just hook it up and mount a servo with a string to the throttle and let it rip.
This might sound like an insane idea but really a nitro motor on a robot would provide so much torque and speed that the robots could tear up grass and move at 20 or 30 mph.
They can also apply in differnt robots like the hovercraft i am trying to build, one nitro motor could provide enough lift or drive to fly down the free way at 60mph. (This could be true or not, i am just saying that it is really powerful)
cool but thats a lovely extra $400 dollars for a 2 motor drive chassis 
If you want to make an RC car why don’t you buy a kit or the parts to make an RC and stop trying to make VEX into an RC car. VEX just isn’t really meant for that application. I don’t mean to offend you or trash your ideas, but it seems to me that VEX and speed don’t mix too well.
I am not talking about building vex into an rc car.
I am talking about adding a motor capable of moving a complex robot at high speeds.
Think of a robot armed with a cmu cam that could follow a certain color car down the street autonomously at 60mph how cool would that be. Just think what you could do and the new design possibility’s.
Vex is a steel chassis which is good and that will support a nitro motor and some of the plastic pieces would need to be replaced but other than that it is a fully plausible idea.
That is what i am talking about, i am not trying to turn vex into and r/c car rather something that can get in and out extremely fast and outrun a human.
Not to mention with the torque these things can put out; if you put tank treads on it and put it at full blast it will make the robot wheelie depending on the configuration of the treads but thats just sick.
Sry for double posting.
I understand that vex was not originaly created to go 60mph but it is possible with a steel frame and some other simple tweaks.
Although a 10lb robot moving at 60mph would cause some nasty damage if it crashed so i guess its not that safest thing to make because at that speed you would probably bend your frame and pray that the bulk of the components are in tact and not smashed in pieces.
And who ever said the nitro motor could only be used for speed? why not a crap load of torque?
Imagine that!
I don’t think VEX will mix with high speed at all. However, the torque may be a nice thing to add (who knows). The gears’ll get worn out pretty quickly though…
yes i think something people have not realized is that making the motors way more powerful (speed and torque!) would mean having to revamp the vex system, metal gears, stronger axles, metal clutches not plastic, stronger limit switches, better motors could rip the arms off before you could press the stop button! same for bumper sensors, could easily smash through plastic at 60MPH
Who in there right mind would use a bumper sensor at 60mph?
I mean thats insane, last i checked the bumper sensor actually contacts that wall and at 60mph it would smash into a wall and break everything.
If you needed to stop the vex at 60mph you would need some form of non contact sensor ie ultrasonic or laser that could sense a wall or obstacle and slow down with some breaks to about 5mph. From there your robot could use a bumper sensor although i would prefer to stay away from contact sensors at those speeds and just stick with ultrasonics.
I am planning on doing some cool projects this weekend, i just need to find some time to build these robots.