Maybe you should have kept it in one piece to make the parts for the conversion .
I am looking at doing a similar lathe to CNC, It depends on what the final spindle speed you need, if you go with the largest motor you can fit, (fortunatly 3ph motors are smaller that their 1 phase counterpart), I would fit a C face motor if possible and use a 4 pole motor at double the frequency i.e. 120hz.
and use a timing belt, if you want CSF and threading, you will need an encoder on the final spindle shaft.
Post the pic.s as you progress.