Final Update
I bought a brass sleeve that OS makes for the 40B carb to fit into the 50SX. The sleeve fits nicely and I drilled two holes for the retention screws. The assembly fit snugly into the crankcase and all air leaks were eliminated. The best starting procedure is to open the throttle to 40-50%, choke and flip the props 2-3 times. Usually after the 3rd flip the engine pops and ready to run. Flipping the prop another 2 times without choke gets it running every time. Allowing about 20 seconds for warm up, the rpm stabilizes and runs steady at about 3000 rpm. Transition is instantaneous and I tried both a Zinger 24-8 and 24-10. Max rpm was 4600 for the 24-8 and 4400 for the 24-10. Idle was very reliable at 900 rpm. for both.
Incidentally, the 40B carburetor (from a 40/46FX) works great at 65% throttle area as compared to the original 160FX carburetor.