This is what I would do. I'm just boat owner, not a mechanic. I would get a piece of aluminum or steel plate, some gasket material (e.g. https://www.summitracing.com/parts/fel-3060) and some bolts. I would create a block-off plate to seal those water passages on the top of the manifold. I would plug all other water ports (they should accept ordinary NPT fittings), except for one that would have the air inlet fitting.
If your elbow is still attached to the manifold, you'll have to get a little more creative about blocking off the exhaust outlet. Perhaps use a short piece of exhaust hose and round plug made from plywood (doubled or tripled) and held in place with a hose clamp.
If your elbow is still attached to the manifold, you'll have to get a little more creative about blocking off the exhaust outlet. Perhaps use a short piece of exhaust hose and round plug made from plywood (doubled or tripled) and held in place with a hose clamp.
