RE: O.S. .81 VR-M Marine
I'd say that a .45 is the maximum that I'd drop in there, any more would be uncontrollable. That hull size is best suited for a .21 .
Seeing how you already ordered the hull from Tim, I'd go with a good .21 as it will be a lot cheaper in the long run for not only the engine and hardware but the fuel costs. If you don't already have an electric starter you can look for one with a pull-start. A decent setup will put you in the mid to high 40 MPH range with no trouble.