Something to keep in mind is that it will likely be
much easier (and cheaper) to secure a 3-phase motor that could be turned into an RPC than it will be to find a motor with the proper mount for the Bridgeport. Of course, some sort of adapter could be made, but that may or may not be easy ...
Should you decide to explore the RPC route, keep in mind that your needs are very modest at this point. To run the 1/2 HP motor on your M-head, you'd probably want a 1 HP idler, and might could get by with less. The good news is, motors in this size range get scrapped out all the time. I've got two or three sitting in my garage, acquired as free scrap, and if you happen to be headed to NC sometime soon, I'd be happy to fix you up - but you can certainly find options up your way as well.
Since an HVAC shop is a great source for free / scrap contactors and capacitors, you might see if such a shop has any scrap 3-phase motors as well. The HVAC shop from which I got all the contactors and capacitors needed to build my RPC only dealt in home installations, so didn't have any 3-phase - but I could have had all the single-phase motors I ever wanted, all for free. The 3-phase motors I
have acquired have come either from the scrap bin at a nearby university (fortunate to have the necessary connections to be given permission to scavenge) or from a friend who used to do repair work on commercial office equipment, some of which was 3-phase. I'd guess I've picked up half a dozen 3-phase motors at one time or another, mostly 1HP or less, probably about half of them in working order. I missed out on a 10-HP motor that was scrapped out - I saw it and went to get help moving it, but someone else beat me to it.
But I did manage to snag a 3 HP 3-phase motor that had a broken end bell - I brazed it up and it is now the heart of my RPC that runs my 2HP J-head Bridgeport.