A well-made aluminum piece that is straight, combined with good gaskets, will seal very well. But it isn't hard to straighten the stock sheet metal, either.
Using a standard body hammer and a dolly (or small ballpeen and makeshift piece of steel as a dolly), and a little patience, the covers can be straightened and the flanges worked back to flat fairly easily. Check with a good straight edge to assure flatness, and that the cover is straight. It's important to pay attention to the bolt holes in the covers, since they are usually depressed somewhat from the mounting bolts being over-tightened over the years. A ballpeen hammer is a good tool to gently knock them back to shape.
If the parts can't be saved for some reason, it shouldn't be hard to locate used replacements. A parts-wanted post in the Swap Meet
forum of this group should turn up usable pieces, as another alternative.
Hope this helps!