I used to own NS26C #143, so that boat’s set-up was similar to yours. You don’t need an OEM muffler. Your engine water pump should have hose connections to the cooling system, but also an inch hose that injects water into the exhaust system before it connects to the hose which leads to the muffler. Just about any off-the-shelf “water lift / wet exhaust” boat muffler will work for you, as long as it’s sized to match your exhaust hose, and takes account of whether your installation is above or below the water line.
Thank you, Bob. That was very useful. My boat’s exhaust arrangement is slightly different to yours but when I checked it out this weekend I could see it worked in the same way. I’ve also seen quite a few recent comments about these mufflers in the on-line discussion. Thanks a ton for your help and all the other contributing nonsuchers. Greatly appreciated by this newbie.