You could go the cheap route and use a couple 750 sized boat motors. A significant gear reduction would be necessary, but I'm pretty sure planetary gear boxes are available.
Have you seen these? ScooterSupport.com They have 36/48V 1000W models; 4" diameter, 6" long, 11 lbs. , ~$160.
That would be nice. I'm tryin to go really easy on the wallet though. Over the last 2 months ive spent about $2000 on rc's and when you're 15 that's a bit too much. :)