Morgan,
I would definitely recommend you keep the 3 phase and get a VFD. The advantage of this is that you have infinitely variable speeds, and can change on the fly. This means, among other things, that when facing a large diameter piece you can start slow and speed up as the diameter reduces. The motor is also smoother.
I Replaced the 0.5 HP single phase motor on my S7 with a 1 HP 3 phase + VFD (package from Newton Tesla) a couple of years ago, and the improvement is significant.
David