I think you may have misdiagnosed the problem. High and low are selected manually on your machine. Can you give it throttle in neutral or park with no problems? Does the machine move or is it drivable when in high or low? Will it move in reverse?
It don't have any "gears" as you call them. If it blew a belt it probably has pieces of the old belt wrapped up in one or both the clutches jamming it up. You'll need to take the cover off and clean the clutches. They may even need to be disassembled to get the old belt cords out.
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