It is pretty rare. I was looking to convert my Optima to belt, but the gearbox was nowhere to be found. A fellow Optima owner told me he succesfully converted to belt drive without the special gear box. The tension on the belt is mostly on the bottom half, the upper half tends to be more loose, so it may not rub against something even without the special pulley, which was the main reason the special gear box was invented.
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