Dave,
Sorry, I thought you mean’t Silver Mist (senior moment! – Mist, Fog….)
The construction of Moonmist was covered in the Model Boats Commemorative Special published last year where Glynn Guest did a complete build. He used a mild form of 5 pole 385 motor as it is important not to overpower a model like this. The Maplins one you are looking at would appear to be quite similar. Others may be able to suggest alternatives.
I think an M5 shaft would be a bit beefy for this small model, M4 would be more the norm and you can use either a plastic or brass prop to match.
It may be that you can still get hold of a copy of the Special from someone (MyHobbyStore have sold out I believe) as the article is very helpful as regards constructing the model.
Colin