First the carb goes from Walbro (USA) to MVVS (Czech republic) stock position, then it is sent back to you in the USA.

What type of carb is on now? it should be a WT 561, which has a large 13.49mm venturi bore.
Earlier engines had only a 9.5mm venturi, so Morris may have sent you an old type carb.

The engine was very well behaved with this smaller carb, so I do not understand why you cannot fly.
To choke if you cannot use the arresting system: Just take the choke valve out, but leave the shaft in. Then you can choke with your thumb.