11.2mm would be ideal. This would be a Walbro with a "28" cast into the venturi. The usual prop would be a 18 X 8

They run pretty good with a "24"

With a smaller prop like a 16 X 8 they could use a 1/2" or "32" Walbro

The "SB" in the part number means it is a short block for several different applications