If I may suggest, a 21 size engine will be far too small and you'll be disappointed if you want it to plane, you would need something more like a 52 or even a 61 for a 46" hull. Moreover with the monotonous high pitch whine they'd also sound and therefore look pretty un-realistic. With an abundance of highly efficient brushed or brushless electric motors now on the market, electric is really now the way to go. Although initially electric propulsion is a bit more expensive to set up than I.C. electric motors are now equally as powerful as I.C.,if not more so, and there is a size to suit practically any model. They are also extremely reliable and convenient and due to environmental issues you are going to have a far greater choice of sailing locations to choose from and with the bonus that your model will always stay clean!