[EHaskins]

If you want to retrofit a go-kart, you could pull the drive and controls from a used electric golf car. Most electric golf cars run 24-48v, 3-5hp motors, and have max. speed of ~20mph.

I know you can pickup used golf cars very cheap, if they have bad batteries.

If you wanted to spend a little more you could replace the analog controls, most older golf cars use, with digital controls.