If you go down the FPGA route you at least can get the accelerator... DSP accelerators for machines that didn't come with them are rare as hens teeth otherwise right? Also ... you could implement those old DSPs in the FPGA... that might even be a good option dunno.

Regardless I'm gonna pick up Terasic Cyclone V GX board... and give it a go probably won't turn into a Mac accelerator but it might turn into a custom Sparcbook if I can get the code from temlib.org working on it.