Can you access the belt? If yes than if you put your belt on in a figure eight it'll rotate the opposite way.
Not really, the pillow blocks and motor are bolted down with the belt very tight so it wouldn't be easy.
When it starts to freeroll, I power off the motor and reverse the pulley manually then jam a paint stir stick in to wedge some grains in the rollers... It usually starts milling again.
If I had to make another one, I'd def start with a better mill that has a 1/2" shaft.
I'd probably use this motor instead since it's already wired and has the correct RPM:

GEARMOTOR - About 80 USD
HOLLOW OUTPUT SHAFT SO IT CAN BE USED FOR AN AXLE THROUGH
OUTPUT IS 190-200 RPM
0.2 HP APROX (150 WATTS)
Uses 12V DC