Had an Atomic 4 that used IP through the first foot or so. Iron pipe and a judicious mittenfull of refractory glue.
Tap the exhaust manifold so there are something like threads, screw in the iron pipe using refractory glue as the sealant, install any elbows drilled to accept the raw water cooling nipple (angle it so you don't flood the engine on a hard start)
Warning, this is not a great solution as the possibility of a gas leak in the cabin...... probably will not pass an inspection to sell the boat.