If you make a part as a "replacement" you will have less of a problem. It replaces a part, it is not the part. As far as sheetmetal if it is not the exact whole panel, it is not that part. That being said, anyone can start a lawsuit. If you make the part with manufacturers approval. They will list it as an approved part, meaning that it is up to thier engineering standards ect. It also may make you "the" supplier.