Here's a link to your clutch,driven and belts- This tells/explains it !!!
http://www.*****************/asymapp.htm
I too think the TEC motors are durable/solid power plants- I've had many a TEC outlast a clone and run well beyond the 500 hr. life span with only regular oil changes. When a TEC motor is running right they have plenty of torque, you're already on the winning side with the manual choke version carb for easier maintenance and starting ability.
I know your son is only 10 and you want to keep the speed down at first- he will be looking for MORE very quickly if he has any common sense and motor skills. A test drive with you in the kart will give you the feel for torque and speed- Every kart is slightly different when it come to speed and torque. Tire size, sprocket size, body wieght all play an important role in how well it performs on your terrain.
I suggest ************* for parts.