If that's the long belt that runs full length under the tractor, would use something like 1/4 or 3/8 dia rope to more closely approx the belt cross-section... and hope that the belt dealer will let you change out for a bigger/smaller size a time or two to get it all right. And if you can get one of the pulleys loose that the new belt will be riding in, take it with you - it will help the dealer to get you the right cross-section size.
If all else fails, got to the Cub Cadet website - find their corp service dept ph # - call 'em.