Jim I prefer the Walbro carbs. Easier to get parts or new ones, also never had much luck with the Zama. Most here use Walbro as well as the players in the RC engine arena. I would use a Walbro WT-289, 324, 391, or 433 if it were my engine. These are carbs I have on hand here that are 11.11mm venturi, all have 28mm intakes, chokes and 2 needles. Any one like these will make plenty of power for you. Tons of Walbro information here and at www.walbro.com
Click on the Service and Aftermarket and check out the links.