Andrew
a quick comparison between the Hurley 22 and my IF Ingeborg, makes it appear that the Hurley's righting moment will be about 80% of that of Ingeborg. Ingeborg's mast is 150mm with 5mm wall. A mast of 140mm should give 81% strength.
Frankly, this is not rocket science. Ingeborg's mast appears to be strong, without any sign of stress when I sail her.
If the project were mine, I would look for anything between 120 and 150mm.
Arne