Very few raw water systems in narrowboats directly cool the engine - this is asking for trouble if anything is drawn in and blocks the system. Most use a heat exchanger, usually built in when marinising the engine, but you can get them separately here and several other places, I imagine. This would probably be the most expensive part. You would also need a strainer, of course, but these are widely available. And also possibly a mud box, although I don't have one.
ETA that I see that's an American site. This might be better. My goodness, they are expensive.