Nearby, in A single board computer using Z180, @oldben wondered about a 6809 SBC. Well, it’s not an SBC, but there’s activity in the rc2014 world:
And here’s a project featuring one of many 8 bit micros:
https://hackaday.io/project/163669-generic-8-bit-processors-prototype-g8pp
Not sure if any answer to the question of OS-9 though.
Is OS-9 from that era even available? The CP/M sources are widely available, which makes them readily ported to modern systems. I just don’t know the status of legacy OS-9 codes, or even FLEX for that matter. (Pretty sure there’s some kind of “open” FLEX stuff floating around).
The uniflex archive is up today.
http://retro.co.za/6809/UniFLEX/index.html
Now to match hardware to the 6809 software.
Get it while you can.