Just get a red plate carb and cam combo with a .880 pipe thats been worked by dover, vause ,or one of the reputable builders. All are easy to install. Tell your kid to rev the motor during cautions so u don,t foul plugs. As he moves up to green blue plate just change jetting. The red plate will probably be a 40main/30low. As he moves to bigger plate move down on jetting 39/30 green 38/28 blue.
The key to restricter plate racing is getting the kart unlocked from the track if your on dirt. To do that is to go up in air pressure. I'll give u a example. One race night on a road trip at new track i was at 10lbs in RR and 7lbs in the other 3 tires. Did terrible. I was listening to my competition. The main i was 20lbs on RR and 14 across the front and 15 on LR. Went from 16th to 2nd and almost won if there was another lap or two. Don't be affraid to air those tires up on restricted class. Hope this helps.