I think I'd want to really test the engine under strain before doing the journey. Moving a JP2 powered boat once, I had the silencer/expansion box blow a tube and water started spraying out the exhaust when having to do an emergency stop - wouldn't want that to happen on a sea voyage. Tie up in some really deep water, and run at high power for several hours, or use the journey to the tide to get some proper stress testing done. Obviously ensure you have all you need to survive should the engine fail. I'd be tempted after my experience to have a bit of pipe work available in case of such issues.
If you really want to be sure, have the engine checked and serviced by a professional, but they can't catch all problems...
When I picked up Victoria with her JP2, I ended up breaking ice for most of the journey back, flat out, for 12 hours a day for several days - so I knew the engine was good when I took her on the Humber later that year. By the time I'd got her within a few hours of home in London the head gasket (which I knew was on its way out) went completely! Still ran thought, and got me home, but wouldn't have liked it to do that on the tideway.
Cheers,
Mike