A user on other forum's that goes by "Rakort" also has this mill. He replaced the original motor with a 2hp 3ph by machining the armature to accept the original pulley and milling a mounting base that matches the stock base, IMO he did a great job. You might be able to just re-use your base from the original motor to do something similar.
I've been in contact with him and so far it's still a work in progress. I was able to use my original 1.5hp 3ph motor by hooking up a teco VFD, works like a charm. I'm very glad I learned about VFD's, I would otherwise have spent just about as much adapting a single phase motor that wouldn't work as well. I'm still in the process of wiring up the "remote" controls for the VFD, I have all the switchs and pots for adjustble speed, jog, fwd/rev, and E-stop, I just need to hook it up.