Yes, this carburetor is probably not available. But I,m sure that any nitro R/C two stroke carb of similar size will work. One that comes to mind is the Moki 180/210 engine carb, a very high quality unit by the way.
The main problem with the Walbro is that the carb throat is too large, this. causes a very weak DELTA P in the venturi. The throat area of the O.S. carb is 62 pct. of the Walbro.