Sounds simple in principle except that we're talking about a rectangular shaped exhaust port,(not really square as such) and often with a bridged section down the middle of it.
I was just trying to keep my answer simple....
If you wanted to know how I would go about the problem myself.... I would make a pattern and cast a part! Simple fix!
Now in saying this you to can cast the part at home.... it's not that hard.
The hardest thing will be sorting out some casting sand... and thats not that hard!
'The simplest of all possible sand molds is a hole or depression in sandy soil. If the soil is sandy enough to resist the hot metal, dry enough to avoid steam explosions, and wet enough with enough binders to maintain its shape, then the metal will accurately take the shape of the depression.
Pouring metal into sandy soil is undoubtedly how the process of sand molding was developed by our primitive ancestors, but it is not a controllable enough process to make much use of in modern casting. Instead of using naturally occurring sand, we need to make our own with desirable properties that can be controlled and maintained. The process is simple for water-bonded sands, which are the most accessible variety for most hobbyists. (Oil bonded and resin bonded sands achieve the same thing in principle, using more complex binders for improved properties.)
To make any molding sand, the first step is to get good sand. Clean white silica sand of approximately 70-90 mesh is a good starting point. Most hardware stores sell a sieved and cleaned “play sand” that will work adequately, though a foundry sand will generally have a finer and more useful gradation of mesh sizes. Olivine sand will also work, and it is usually green or black. The grains should be sharp, not rounded, and the sand should not contain rocks or debris.
This sand now needs a binder. For water-bonded greensand, the binder will be bentonite clay, approximately 10% by weight. Bentonite is available in high purity from ceramics suppliers and in bulk amounts from well drillers and some farm supply stores. Some types of cat litter are bentonite chips, but these will have to be ground into powder before use. A coffee grinder works well, if slowly.
Adding more clay will increase the sand's strength but decrease its porosity, leading to gas defects in extreme cases. Adding less clay will lower the strength but make the sand more porous. 10% is what I prefer, but it is possible to get away with as little as 6%. As long as the sand molds well, less clay is generally better.
The clay and sand will not become greensand without the addition of water. Water should be added slowly to the blended clay and sand, and thoroughly mulled in until the sand feels uniformly damp (but not sticky) and forms a cohesive ball instead of crumbling when squeezed. It takes time for the clay to absorb water, and mulling action is necessary to distribute the clay evenly (but excessive mulling will “wear out” the sand by rounding off the grains, reducing porosity and strength). Sand generally improves dramatically if it is left to sit overnight after mulling in a sealed container, and the sand will continue to improve even more for several weeks after it was first made, as long as the moisture level is maintained.
Too much water in the sand increases the amount of porosity necessary to prevent gas defects and will cause the sand to stick to things. (Dramatically too much water can even cause metal to shoot back out the sprue of the mold from the steam generated! A mold that does this will never produce a usable casting, so it is best to retreat for safety's sake until the metal has cooled.) Too little water will cause the sand to abrade away and lose strength, eventually crumbling. A sand mold can be dried after molding without a loss of strength, but the drying will cause shrinkage, which can lead to gap run-outs and problems with dimensional tolerance. It is generally better instead to use properly conditioned sand and to pour molds as soon as they are made.'
Some of these words have come Alloy Avenue metal casting forum.
I think this lot may need to be moved to its own thread.