A 3/4 hp motor puts out 560 watts of power. The electrical power in will be somewhat higher. At 24 volts, that works out to around 23 amps. 3/4 hp is not a lot of motor for a mill so I would expect that you would be loading the motor fairly well and need the better part of that 23 amps.
As an aside, if the motor is not a permanent magnet type, you should be able to run it on ac. You would then just require a transformer which is easier to find than a dc power supply at that current draw..