A gear motor would be the best choice for that type of vehicle so the top speed could be kept in the 6 to 8 MPH range. If a standard motor without gear reduction was used then the top speed would be too high.
For a 100 pound loaded weight vehicle running on relatively flat ground at 6 to 8 miles per hour a 250 or 350 Watt motor would have plenty of power.
Our 24 Volt 350 watt electric go kart power kit would be a great choice for your project. It is available on this page: http://electricscooterparts.com/electric-go-kart-kits.html
To obtain the desired top speed with this kit our 'top speed and gear ratio calculator' can be used to figure out what size wheel or axle sprocket to install. This calculator is available at this link: http://electricscooterparts.com/motorwheelgearratio.html
We would be glad to help with the gear ratio calculation and determining what size sprocket to use if you would like.
The parts in our kits can be changed and added to upon request, so for example if you like the electric go kart kit however want it with a different throttle, or different size batteries, or with a wheel sprocket and chain then we can make a modified kit to your exact specifications.
Please let us know if you have any questions.
Sven Lemming
I built a replica F14 for my twin grandsons. It weighs maybe 50lbs, and maybe 100 all up (with both twins on board). I'm wondering what motor and gearing combo I'd need to have the kart go 6 to 8 mph, on grass playfields.
Thanks for any help offered.