A burke #4 was my 2nd. mill in the early 70's. It was a REAL machine capable of doing real work compared to the Atlas. I'd have loved to have found a vertical head for mine at the time,but this was ages before the internet. I fully recommend the Burke #4,and it isn't so heavy that a couple of guys can't move it about. I took mine off a tall flat bed trailer with a friend(but was a LOT younger back then!)
I saw a vertical head for the Atlas years ago. It had a little round leather belt like an old sewing machine. A small pulley was held in the spindle of the mill to drive it. I don't recall seeing any way to tighten the ever stretching leather belt. It did not look like it would have enough power transmission to knock your hat off. These were not made by Atlas. Someone else made the vertical head and a slitter for Atlas mills.
I'd certainly make a way to mount a motor with a V belt on such a head,if I were you.