What I did was to cut the coveralls in half across the waistline. First I zipped up the zipper, then I cut.
Then I sewed off the end of the zipper so that the top half can be used as a jacket. I have to pull it over my head, but the zipper still works.
Tyvek doesn't require you to hem off the edges so I just left the bottom edge of the jacket as is. But I sewed off the edge where the zipper connects to the fabric so that it wouldn't fray.
Next I took the leftover part of the zipper out of the pants and sewed up the fly. Then I folded over the top and hemmed it off so that I could slip in some elastic for the waistband. With the waistband in place, I figured out how long the pants should be and hacked them off to that length.
I also have a really nice hooded waterproof-breathable rain coat and regular waterproof-breathable rain pants. The kind of thing that ought to withstand a heavy-duty rain storm. But they seem awfully hot.
Now I have a choice between the two suits or a combination of the pieces. My Tyvek rain suit weighs practically nothing, but it's a disposable suit so I don't know how long a life it will have. Definitely I will bring the pants. I will have to decide if the Tyvek jacket is adequate. It probably is, but I'm not sure yet.