ORIGINAL: j.m.
You would be much better off closing the hole in the bottom, and using a descent hardware setup and a surface drive. It would be MUCH easier to get the speeds you are looking for that way.
That would be three holes to close, as the hull originally has three 540, each turning a prop, running on 8-10 Sub-C, if I'm not mistaken, with a mechanical two step switch...
Basically a surface piercing drive is faster, the work needed on the hull is about the same, so the choice is up to Brodjack.
Regards, Jan.