Re: Hi from down under
The smallest carb I can find here for the engine I want to use is a 10.5mm Walbro. That equates to 2/3 of the cross sectional area of a 12.5 version. It wouldn't alter low speed torque, but merely cap the top end and peak power.
The other question is what's available in small cc, like the 22cc petrol stand on scooters we get here, or 25cc garden equipment engines? Small cc is probably the easiest way to have a low output in the first place.