A belated reply....
I had one of these machines but sold it on.....
I fitted a 3 ph motor and a vfd to mine before it went. I retained the original pulleys and mounted the motor behind the mill on a plate with slots for adjust the belt. Sizewise I think I went for 1/3 hp...but of course a 3ph motor is probably a but more efficient than its 1 ph counterpart and smaller too boot.
As for lead screws, well mine had been altered long before I got hold of mine....I believecthey had a rack and gear fitted for quick return on the X axis, mine came without....it was fitted with a lead screw and I believe it was 1/2" BSF. Now whether that was an original I dont know...btw that happens to be 16tpi, I wasgoing to go with a cycle thread for 20 tpi but never got round to making a new leadscrew.
I'm sure you will be able to sort something for yours