The PowerPC 7457 was built on a 130nm process, used in the AmigaOne XE as well as the Apple G4 machines. That's probably about as good as you will get for an community-led chip fabrication. You could probably get it up to over 1GHz if you made a 68k at this size. A modern FPGA is probably the better way to go for this kind of thing. I doubt this free fab includes things like die testing and packaging. That's an expensive process so someone would need to front some money to actually get the testing and packaging done for enough chips to make the cost actually worth it. It would be much cheaper to design an interposer board that could plug into a motherboard to take the place of the original 68k. This would also allow you to continuously upgrade the processor without requiring a whole new fab to take place.