Even windows 3.1 is not open source as far as I know.
Sadly windows 7 won’t even work with most good CPU’s made in the last few years. Microsoft stated forcing windows 10 around the time the 7th Gen Intel processors came to be. They did the same with AMD.
Get a mobo that’s compatible with Win 7, and figure out the minimum number of drivers that you need. I’m running a Ryzen 5 with Win 7; it was kind of a pain, but there are sources online you can check to see how to do it.