Do I need to replace the rotted header under my deck, or can I simply insulate and cover it with aluminum flashing and siding?

So, I’ve got this two-story house with a crawl space foundation, and I noticed that under the vinyl siding where a deck used to be attached, there’s some damage. It looks like the header from the floor joist has rotted away in a few spots. I cleaned it up and it seems like the outermost part of the header is completely gone, and in one spot the inner part is rotted enough that I accidentally poked a hole in it. I spent a while cleaning out the damaged area and now I’ve got some fans running to dry things out. I’m also thinking about treating it for termites and other bugs.

To fully repair the header, I’d have to take down the deck, but I’m wondering if it’s even necessary. I think the load is being carried by the sill and floor joists, so maybe the header isn’t doing much structurally. Could I just stuff it with insulation, cover it with aluminum flashing, and replace the vinyl siding without replacing the header?