I have an 86' SMB1 with BX control and 4400 hours in very good condition. The machine is still booting off of the original tape drive and I live in fear of the tape dying. I have been debating doing a retrofit with Gecko 320s and Campbell combo board (left over from another project). The mill is equiped with Electrocraft permanent magnet servos and optical encoders, and I know that the encoders at least will work with the geckos, and I suspect the servos will also. I also have all of the original wiring diagrams so I should be able to patch into the main din rail connectors for a fairly simple wiring job. My other option would be to buy the upgraded controller from amtsinc.com and run it with a dnc connection.
I had good luck with mach3 and geckos on a retrofitted bridgeport, and I have a post that works in my cam package (sprutcam). I am really leaning this way and I would like to make this machine useful again (trust that it will boot and run), and if not, get it out of the way. If anyone has any comments on this, please let me know.