There are a number of manufacturers such as Dings, Stearns, and Warner Electric who make electrically operated models in both DC and AC. One popular style is to place the brake on the NonDrive end of a motor, however this does require that the motor was designed to accept an aftermarket brake. It may be possible that the motor manufacturer may already have a brake "kit" for your specific motor.
Another brake design has a C-Face machined on both sides of the brake and the motor is placed onto the DriveEnd C-Face of the motor sandwiching the brake between the motor and machine. These C-Face dimensions follow industry standards from Nema (National Electrical Manufactures Assoscation). You mentioned machining already done to mount the motor, I can only assume that this is a Nema C-face.
Your easiest bet is to use what's called a failsafe brake, electrical power to the brake holds the brake off. Wire in with the motor, this way when you power the motor, the brake comes off. Disconnect the motor from power and the brake comes on.