A GX200 carb should do, we use a tuned GX200 for training. Sharpens braking skills etc. and allows the lad to hunt some of the other classes (adult novices etc.) in the often mixed practice we have.
Makes about 9hp compared to the 7hp of a very good GX160. You might find the 8 degree key a bit much, it's normally used for the GX200 which is 5 degree retarded as standard compared with the 160. The biggest gain we saw was putting on a completely gutted exhaust, that alone was worth nearly a second on a 1.05 lap. Mighty loud though, so depends on your circuit.
After that you want to look at flow both in the head and the air side of the carb. If you run the engine left size, just buy a filter adaper and K&N style filter. For us with the engine on the right, I was able to convert the standard filter support into an elbow for the K&N to clear the wheel.
In the head a bit of mild porting will help, radius out the angle from the valve pocket intersection and just generally clean up. Polishing is alway debatable due to the varied effects of turbulence, but if you've got the time it's very satisfying.
Finally, most tuners seem to agree that 9hp is at the very limit for the standard rod. You should be getting close, so the billet one might be worth considering.
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