I've been in this scene for about 2 decades. I managed to dig up source code for a lot of the one-off ROMs that I've made over this time, get them building on a modern *n?x OS (without any steps on MS-DOS), make the minimal changes to get them working correctly on hardware or in a modern emulator, and collect them as exhibits in a repository. Most everything from 2007 and later builds in ca65; for the x816 projects, I had JRoatch help me use ASM6 as a substitute for x816.