Find the tooth count of the belts you are using. Count them to verify. use a white market or a piece of tape to note your starting point.
timing belts for D series, I believe, are in range of 102 to 108 teeth.
water pump pulley size and timing belt tenioner pulley size make the difference in what you need, next to deck height.
I would assume a d16z6 belt with a d16z6 timing tensioner is the combo you want. You will want to grind some clearance on the timing belt side motor mount if you find the timing belt being too close to the mount for comfort.