Cast aluminum absorbs fluids like a sponge. I've fixed broken mounting ears on Holley base plates in the past. Wash, clean, acid, v the edges, all of it, and when you strike an arc and make it melt, it smells like the exhaust of a running engine - strong enough that you don't want to breathe it. Water pumps and housings - like exhaust from something with a blown head gasket. Perfectly clean motorcycle cases - like oil on a hot exhaust.

One trick is to use a TIG torch, strike an arc and work the area of the crack as if you were going to weld, making it hot enough to melt without adding filler. Contamination will come to the surface and can be cleaned with a wire brush, carbide, etc. May have to do it a number of times, but you should be able to get it cleaned to the point where you can add filler and get it to flow and freeze without looking like puke. If it doesn't look like chrome when it melts, it has junk in it. Propane works to preheat and clean the surface, but it really doesn't cook the contaminates out of the casting. The quality of the casting has a lot to do with it also. Some castings are made from garbage and turn to dog poop as soon as you make it melt. Aluminum can run smooth as silk or it can turn to junk and make you say bad words. It just depends.