I have been upgrading a couple of old Amigas with accelerators. You can get new 68030s boards running at 45/50Mhz (there are actually TWO different providers I'm aware of) with gobs of memory, options for drop in and card slots, etc. You can also get a FPGA based "68080" drop in replacement CPU (I don't have one of these).
On the mac side... Novy Quik30s and Total Systems Geminis appear to be unobtanium. Options for accelerating compact 68K macs are few are far between.