After buying the correct belt (6 ribs, 120.3" long, Gates Micro-V AT K061203 ~$52 at O'Reilly), I was able to locate the idler pulley, drill its hole, and space it out from the thin prototype bracket properly. Since this
idler pulley w/spacer & bolt was from the JY, I cleaned all the accumulated grime out of the backside so it would vibrate less. I also placed it intentionally close to the edge so it overhangs, making future cleanings easier without removal.
Turns out this pulley has a 10mm bolt - the bolts I'm finding now are 12mm, so I'm redrilling & tapping the brackets to match the particular bolt that I ship with that bracket.
