Something sounds fishy when an aluminum part is said to be mangafluxed. I may be mistaken, but I believe the material to be magnafluxed must be able to be magnatized. Aluminum can not be magnatized.
Also, a casting flaw is not always acceptable. If there are cracks in the part resulting from a casting flaw, they are still cracks, and cracks are not good. These cracks may amount to a negligable hazard, if they are minor surface cracks.
I would assume a factory representative saying the product is safe should be acceptable.