Is it possible to make a friction style primary drive with a link belt instead of the traditional V-belt? You know the type with a tension pulley that lets you engage or disengage the primary drive.
If you're describing what I think you are, then the engine idles by allowing the belt to slip the clutch. I am not sure if this is necessarily the best setup, because the belt will wear faster this way. But it should be able to be done.