Scag Walkbehind pulley for Hydro Pump belt

[MOOSE]

Ok, here it goes. I have a Scag 52" Hydro older one, 1996 I think. The pulley on shaft of motor for the belt going to pumps keeps going bad. What do I mean by bad? The pulley keeps coming loose and will move on shaft(up,down & around). I have replaced it twice already but last time I cut the key in half for the pulleys. So when it come loose this time it moved the key back & forth and messed up the shaft. So I gotta replace motor. Oh well!
My question is when placing new pulley on new motor how do I get this thing to stay put. There is 2 set screws on pulley but they seem to be coming loose.

[Restrorob]

I had the same problem with the motor pulley on my BIG air compressor.
I went to the hardware store and got a couple square headed set screws. I then snugged the one down over the key just to make a imprint where it hits and ground a flat notch in the key. This moved the V tip on the set screw down out of the pulley threads for more thread usage then used loctite 246 on the threads. You can get better torque on the square head than a Allen wrench.