I now have the engine installed in the plane and I started it up this morning. It will start (with some difficulty) and run, but still not well. It will only run with the low speed needles open 1/2 a turn and the high speed needle fully closed. Even then, it runs rich at low to mid- range throttle.
It seems to me, that the lever now needs to be bent a bit more to reduce the amount of fuel being supplied to the carb.
The best advice I can find about the fuel metering lever setting is in this WB manual. Walbro advises that there is no manual for the WJ model!
As can be seen in the screenshot from the Walbro WB manual, the lever is set from 3/64 to 1/6 inch. The setting that I now have it is slightly more than this, and it is still not possible to lean out the mixture at any range other than idle speed sufficiently.

I am hoping that one of you Walbro experienced people will provide a comment on the above.
From the Walbro manual:

Edit:
By mechanic buddy Barry has just called in and we collaborated on the next change to the engine/carb/exhaust. One thing he mentioned is that the big exhaust (that is very noisy) might be offering too little restriction. Tomorrow I will fit the original chainsaw exhaust and give it a go without changing anything else. If it is still the same, I will adjust the fuel metering lever down further. The engine is obvious getting far too much fuel. But we agreed that one change at a time is a good strategy.