I picked up this from APSCusa and my son loves it.
http://www.apscusa.com/New-Magnum-12...art_p_568.html
It has 3 forward gears and reverse. goes like stink. I would not want my son ( he is 5) in a 13 hp kart. I replaced the belts that came with the kart with 6 point harnesses. Also I got him a HANS device. The kart is very fast. Kids necks are not very strong and the helmets they wear are very heavy in relation to their bodies. So neck / spine injuries are a concern, hence the HANS. LEATT makes a nice neck restraint and it isn't $500+ like a HANS. As speeds increase so to must safety.
(My Bro in law is an pediatric orthopedic surgeon at a children's hospital so we are very safety orientated)