I'd suggest researching diy electric skateboards. Check out
http://www.electric-skateboard.builders
I'm building one now and it should go roughly 20mph but there are a lot of people on there that are going 30-40mph and weigh anywhere between 150-210lbs. That forum is really helpful and talks about electronics, mechanics, etc. It may be overkill for the weight but you could go in that direction and tone things down a ton. If you search the forum there are calculators to figure out speeds based on your batteries, motors, weight, current and gear ratios.