That DP appears to use the "swinging gate" motor mount, so fairly easily fabbed up. There should be a corresponding hole to the one in your top picture, (high and to the left) and that stud sticking out the back in pic 2 is the tensioner for the belt.that simply holds open the "gate",and the screw on the side keeps it in place.
I have a newer version of a Rockwell, so here are a couple of pics of its mount:
If you just want a working DP, and not a "restore" (Sound like it to me), most any 1/2" 1725 motor will do the trick. a 1040 would be better for getting slower speeds. if you plan to drill steels a lot, you need to go slower still for larger drill sizes. >3/8" say)
You can do an intermediate pulley set, or go for a variable speed. Treadmill motors are great for this.
Get a cast off treadmill (Often free or cheap on Craig's List), and scavenge the motor and the last PC card in the control circuit (it will have a heat sink, and wires going from it to the motor, possibly through a Choke that resembles a transformer coil. those two or three items are all you need)
With that you can fairly easily have motor speeds from a couple of hundred RPM to about 6000 RPM.
The only hard part of a treadmill motor is the drive pulley. That needs to be modified on a lathe to fit, or you could run the original treadmill drive to an intermediate pulley and regular V belts from there. or a last possibility as to convert the spindle pulley over to the Treadmill belt type.