An early version of the Mac OS ran on a lightly modified Lisa; the product was called an Macintosh XL and included a hardware mod that changed the original oblong pixels to square pixels. Its 68000 ran at a blistering 5 MHz.
If you're serious about running a 68000 on an Altair bus, you might want to go with the somewhat castrated eight bit version of the chip, the 68008. Hard to find, though. Its address bus was cut from 24 to 20 bits while the data bus was cut in half from the original eight bits. I have a circuit for this, and let me tell you that you're going to have to hijack a few unused S-100 lines for getting the 68000 bus grant and DTACK scheme up and running. Also (and I may be wrong here, it's been a while), the 68008 was an NMOS design and had a minimum operating frequency requirement. Hopefully it is less than the 2 MHz upper limit of the rest of a typical S-1000 system.
I always had a fondness for the 68000 family, even though it really wasn't fully debugged until the 68030 (Integrated MMU) plus 68882 (FPU) combination. You see, I had learned Unix on a pdp-11/70 and the 68000 family is the logical 32 bit extension of the good parts of the pdp-11 CPU design. (Compare with the VAX-11 CPU that suffered a design by committee approach.)
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