Bruce, if the correct belt is no longer available there will probably be other people having this same problem. There seems to be plenty of room in the SB590 side compartment between the upper and lower pulleys, so using a slightly longer belt and having the idler adjusted to be permanently in contact with the back of the V belt shouldn't do much harm. The idler seems to consist of a sealed ballrace, so it should last many hours even running all the time (which it will be now, but wasn't intended to be). Even if the ballrace had to be replaced eventually they should be readily available (unless it is another discontinued part, like the original belt).
