Hi Steve, I hope this is some practical advice. The Ben Ain is a largeish model, but a sedate one. I imagine that you picture it steaming gently across the pond. Whilst I totally endorse my fellow posters love of the T 12 motor, you have a perfectly good MFA motor that I know from experience is a good product. My only concern is that 6 to 1 gearing is rather high for your model, I would have chosen a lower ratio.. That said, if I were you I would install the motor, use 7.2 or 9.6 volts and see if you are happy with the performance. The motor will turn the prop without effort, just not very quickly. I think you will be happy with the result, if not come back on here and ask again, but I wouldn't go out and buy another motor until you have tried the mfa one. I'm quite an old modeler and have built many models with geared drives, but still get surprises from the efficiency of a large prop and good gearing.
Chas