If at first you don't succeed, try, try, try ten times again...
I am not sure why such a simple project took me so many times to complete. So I finally got the idler the correct size with the Gates machined pulley. The original pulley is smooth with a 3mm bolt shaft. You are not supposed to use a flat pulley on the tooth side of belt. I used the M3 bolt, but with a 6mm aluminum spacer. Works perfectly and will keep the teeth fresh.