Digit

I don’t know if this is the sort of answer you’re looking for (or if even all that pleasantly viable with no GPU), ~ almost certainly not what you set out looking for, in fact, but, it’s the only answer springing to mind [Edit: Except also flare]:

Pioneer Space Sim

It’s open source, actively developed, and lots of opportunity to poke around in. Although not just a game dev engine, still may be a fun educational and productive experience.

An oddball answer for consideration on the side at least. ;)

Expect idiocy.