Hi thanks for the intro john .it looks to me your machine is a colwood ra model which seem to be more common in Australia than some of the earlier models your engine doesnt look to far wrong and I believe should be a mk 12 Villiers.
I currently own five colwoods .4 model a variants, and an early b, but might be acquiring an ra soon , I also have various tools including a sickle cutter bar , , and a generator attachment.
Plus I also run the colwood register .
Feel free to pm .
Best
Steve.