Update:
I bought a O.S. 40B carburetor as used on the 40 & 46 FX. The throat is 13.5 mm while the hole in the crankcase is 15 mm so I used thin aluminum shim stock wrapped around the throat to bring it as close as possible to 15 mm outside diameter. It fitted pretty well in the crankcase and produced the same 5000 rpm as with the Xoar 24-6 (cut down to 23-1/4). This time though I can open the throttle to wide open without having the engine bogged down.
I swapped a Zinger 24-10 and the engine runs great at up to 4400 rpm without over heating - ever so slightly rich with a hint of bluish smoke. Reliable idle is down to about 1200 rpm and cold starts almost instantaneous, typically 2 to 3 flicks get the engine running
I am very happy with the performance so far.