you'll love the reliability of the rotax i think (if you jet it properly).
my friend and I ran from 1030 to 4 nonstop today and the kart didn't show any signs of hiccup. This was on an engine that's already 24 hours old.
all that needed replacing at the end before the next race next weekend will be a sprocket and a chain.
just keep everything properly lubed (remember to lube the bearings) and you'll be fine. chances are your son will need a break before the kart does
have fun