I haven’t tried these so I cannot comment on their quality. But this has a list. Of particular note is RetroArch, OpenEMU, and Gens as three FOSS options.
Edit: Also, alternativeto.net is usually a decent source for finding alternatives for specific software. Here’s the list for Kega Fusion alternatives. This has some more options than the other link I provided.
You can add it to steam, it’s free and dlc bought from ea works with the steam version.
I find it’s easier to get the ea version running though, so I usually just add the launcher as a non steam game, and that works fine too.