My impression is most BP Boss conversions are handled in one of two ways:
1) Toss all the electronics, AND the motors. Proper drivers for the original high-voltage steppers are hard to come by and expensive. The motors can be replaced with modern ones for much less money. If you go this route, you also have the option of using servos instead of steppers, for little additional cost.
2) Toss all the electronics, but keep the motors. But, performance will suffer, as the modern drivers (like Geckos) will not be able to drive them to full power, due to voltage limitations. This will, however, give a perfectly useable, though perhaps not fast, machine.
Go to CNCZone.com, and peruse the Bridgeport Mill forum. You'll see every possible option,with pictures, and have plenty of people to ask for advice.
Regards,
Ray L.